I'm curious if some of my sealed tins need to be jarred.
I inspect every sealed tin I get to see if it is ... well ... sealed ... when I get it. If in doubt, I jar it.
If I'm getting the tin at a B&M, I'll simply grab another tin if one of the tins I am picking up is suspect.
Some tins are not sealed by design, such as Solani Silver Flake 100g tins and if the plan is to cellar such tins, and not smoke them within 3 weeks or so, I jar them.
It's very rare, in my experience, to get a screw-top (round) tin with the vacuum seal broken -- only happened to me once in the last 20 years.
Except for the obvious, such as the Silver Flake I mention here, some tins, particularly the "pop top" rectangular 50g tins, such as Peterson University Flake, Dunhill Flake, Erinmore Flake (you get the idea -- the small thin rectangular tins) are more subject to losing the seal. This happens when the UPS gorillas played hockey with your tin, or you dropped the tin on a hard surface, etc.
Properly sealed tins, if left alone and undistrubted, are fine stored in the cellar (any relatively cool, dark place). All opened/unsealed tins, unless smoked up within a month, should be jarred.
If you put a new tobacco in a jar that has had a different brand/flavor will it effect the new tobacco? Do you give the jar a good wash first?
Yes, flavors can meld, which is not always a good idea. I'd take a whiff of the empty jar that had another blend in it. If the previous blend was an aromatic or a blend with a pronounced casing/topping or some other smells that can ghost the "new" tobacco blend you're going to fill that previous-used jar with, I'd wash it in hot soapy water (including the lid), then dry it very carefully and completely before putting tobacco in it.