I'm not sure anything I would list here would count as myths, but they are ideas I treat skeptically and do not employ in my own pipe smoking, despite their general acceptance in most pipe smoking circles.
1. You need to break-in a pipe before you can really smoke it as it was meant to be smoked. You need to build cake, and blah blah blah. None of this is true in my eyes. I keep my pipes clean and cake build up is minimal at best. It's not necessary. What is more important is keeping a pipe clean.
2. You must rest your pipes after every use. You can smoke the same pipe more than a couple of times in the same day without destroying it. I do think there is some value in "resting" a pipe, insomuch that use of any object typically entails its degradation unless due diligence to cleaning and repair is managed. But as a general rule, people treat briar and meerschaum as being far more fragile than it really is.
3. Every new smoker should start with an aromatic. I would say a new smoker would be best served by enjoying a sampler and then gravitating towards blends most like the tobacco he enjoyed from the sampler the most. Aros can be great for some newbies, and not so great for others. I started on aros, and it made my entry into this hobby probably a little tougher than it needed.
4. That the quality of how a pipe smokes is entirely objective. It isn't. Perception of things has a very large impact on how we take to them, regardless of the facts (or lack thereof) present. I'm convinced if you took a 100 veteran pipe smokers, blindfolded them, and had them try to reliably tell the difference between a basic $100 pipe and some custom piece, most would not be able to do so with any consistency. This is not to say there are not objective considerations that impact how a pipe smokes. But pipes are fairly simple tools, and I think people convince themselves their $800 pipe will smoke better than their, say, Peterson or Neerup just because it makes it easier to justify the pricetag. You're paying more for aesthetics than anything else.
5. Pipe smoking is physically healthy, or at least not harmful. We shouldn't kid ourselves. The mental benefits of smoking may very well outweigh the physical detriments, and pipe smoking in moderation helps alleviate some risks. But it is a physical risk. One I'm willing to take. But some pipe smokers act as if there is not physical risk at all, and that is just delusional to me. But to each their own. Wouldn't be much of a vice if it didn't incur some risk, I say. Where's the fun in having a vice without some risk?