I pretty much try to keep my regular flavors in their own tank -- blueberry muffin/banana nut bread in my cartotank, Virginia in one of my kayfuns -- though that may be changing, as I really don't care for or vape that Virginia much at all anymore. My other Kayfun, I switch flavors in it a lot, and it can be a real pain -- the stainless holds no odor, but the o-rings do for sure, and maybe also the plastic part of the tank. So far I've managed, but eventually I may run into a flavor so strong that I have to boil that kayfun to get rid of the odor.
The thing about cartos is a definite +1 -- I have the punched cartos for my tank, but I also keep some regular cartos around, if there's a flavor I sorta like but want to "try on" for a while. I do have a dripper I can use for quick taste tests, but I hate dripping for just sitting around, so the plain cartos are a good compromise.
I'm finding that it's true that using multiple flavors helps to prevent "taste fatigue", a/k/a vaper's tongue, so it really is a good idea to have several tanks setup with flavors you like to rotate around -- also because certain flavors go together (my tea with blueberry muffin), and certain flavors emphatically do NOT (pineapple juice with blueberry muffin, argh -- or drinking tea with a coffee vape -- that one is just WEIRD).
Andria