Altering a Sitter - Bad Idea?

This would probably be best answered by a pipemaker, though I will express my hesitancy to alter a pipe. First, with that really cool looking craggy finish, sanding / carving away part of the briar is probably going to look funny. It would be impossible to replicate that rustication and the stain color, and the pipe would probably just end up looking odd with that kind of variance in the finish. Second, you'd need to consider the depth of the chamber and location of the draught hole in comparison to the pieces of wood you'd intend to remove to be sure you aren't weakening the pipe or making it more prone to cracks or burnout. I'd just keep it as-is. It's a good looking pipe!

I have a pipe or two that I would categorize in the semi-sitter category, with small sitting surfaces on the bottom of the bowl or slightly rounded surfaces that aren't dependable in making the pipe sit. While I have never considered sanding either/any of them down for a flatter surface, that is a plausible approach. The counterpoint to that is that you will not end up with a comparable finish on the bottom of the pipe. You might get it fairly close, but it will never entirely match unless you refinish the whole pipe. I don't recommend that. Since most pipes aren't sitters, you could just treat this one the same way. Get a few inexpensive single-pipe stands and use them, or perfect the art of leaning the pipe or wedging it where it won't fall. Sanding it flat would be fine. Your call, of course. I'd probably use it as is. Little food condiment crockery bowls that are small enough to keep a pipe upright also work nicely and cost little, and can hold one or several pipes, which is convenient. Get crockery rather than any plastic or composite, for fireproofing. Eventually, you might buy an absolute sitter; one I can recommend is the Peterson B11. That one would stand up in a wind storm. I own two.

Get a ball-bearing that fits the bowl and drop it in there when not in use to keep it sitting square.

When I first read your post, I mistakenly interpreted it to mean that you were thinking of reshaping the pipe so as to not be a sitter, since it is not a very good sitter. Seeing now that you just want to sand it a little flatter (as you clearly stated), I'd say that probably wouldn't hurt anything, but it will be hard to fix the finish / stain to look decent. I still stick by my "leave it as-is" approach ::

Lean the pipe against something. Even if you're good with the disrupted finish now, over time as your pipes aesthetic develops, such disparity might become a minus. I have an Ashton Brandy with a diamond shank that doesn't quite line up to the diamond stem. At first I said "so what," but as I found which pipes worked for me, it didn't.

With the bottom rusticated it will not be easy to make it blend in. You could just buy a poker or cherrywood for times that you want it to sit well. The other option is to sand it to smooth and try to match the stain, as many sitters are sanded after rustication or blasting. If you don't intend to sell it, it is totally up to you. But many guys regret altering a pipe. However, if it works exactly as you want it to after, you may not regret it.

Here are a couple of the stands I really like:

https://www.amazon.com/gp/product/B0753HJ36G/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B06Y42ZJTD/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
I have several of each. They both seem to hold a variety of pipes very well but the top one holds smaller and billiard type shapes a little better and the bottom holds churchwardens better.

I've done this several times. I don't care about diminishing the value of the pipe. If I like it, I'll have it forever. I love a good sitter! I am not a professional pipemaker, but I make them occasionally.
First, practice of junker pipes. It is very difficult to get a flat surface sanding manually. You get it flat with a lower grit, and then end up rounding it off with higher grits. So you either end up with a rocker, or you go back take more off. You can easily end up taking off 3/16" and leaving the bottom of the pipe too thin below the airhole. If you can start with some pinch calipers to really know how much you can take off, you will thank yourself. It may already be too thin.
So, go easy. I'd start with 2 single passes of 600 grit, 2 passes of 1200, and 4-6 of 3-4000. A pro guy would probably say it's crazy to do it by hand. A proper disc sander with the right pads would make it much easier to get it flat enough to sit flat. By hand, I have to very very carefully sand out the middle by putting more pressure on the interior portion, so it will site flat around the edges.
After all of that, you will have to restain and buff. It's a project for sure. I would get confidence on cheaper pipes before attempting it on a good one. If you try it on some junkers, you will know if you want to try it on this one. It is risky for sure. I think the reward is worth it, but I modify my pipes all the time (if I think I can improve them). Don't do it if you are afraid to screw it up.

You could get a spherical rare earth magnet, drop it in the bowl, and set it on the lid of a mason jar.

Here I have a magnet on the underside of the lid of the jar and one in the bowl of the pipe.

Yep, but like the magnetic pipe stands, it's not going anywhere. :mrgreen:

You have a nice looking pipe and want to sand off the bottom? Seriously? It will take more than a little "touch up" to make that sit easily. I think you could easily take off enough where it would sit without affecting the smoking qualities. I wouldn't do it but it's your pipe.

My younger brother had a sitter with a wonderful bottom. The thought of taking sandpaper to it never crossed my mind. Magnificent it was in shorts or capris. Yes capris, it was a long time ago. Obviously she made an impression though, a vivid impression.

So you never had the opportunity to alter her, then?

I was in junior high and she was a high schooler. Unrequited love to be sure. She kick started my adolescence though.