You shouldn't leave multiple blends open in their unsealed tins, in my experience.
Small mason jars are certainly preferable to an open tin.
If, for example, you only had one blend that you were planning to smoke through in a short period of time, then I'd say it's alright to leave it in the tin. To delay moisture loss, though, I'd put it in a good freezer bag.
When you get used to smoking a blend, you'll notice the flavour variations as the tobacco dries. In my opinion there is a sweet spot between not too wet and not too dry that is "just right". It will take you some time to find this, and it will not be the same between blends...
Also, if you plan on aging tobacco, you should have it in a separate mason jar that will not be repeatedly opened and disturbed. Everytime you open the jar, at best, you basically reboot the aging process - at worst, you stop it altogether and kickstart a decline. (Particularly if a blend already has some time on it.)
My storage philosophy in short:
-Undisturbed mason jars for aging
-Mason jars or bail-top jars for your daily rotation in the short to medium term
-Open tins kept in a freezer bag (or even well-sealed tupperware) for immediate smoking
*Regarding moisture, I should add that most of my problems related to smoking performance/enjoyment come down to this key element. If the tobacco is too moist, you will experience steam burn/tongue bite/poor combustion/difficulty lighting/sootiness/goopiness. If the tobacco is too dry, you will experience a loss of flavour, harshness, and potential for tongue bite as it will burn faster than you are used to.
The best way to remedy moist tobacco is to dry it on a sheet of paper or paper towel for a number of minutes until it isn't so damp/tacky and the best way to remedy dry tobacco is to rehydrate it in small quantities as you desire it. (There are countless ways to do this - I personally just use a damp square of paper towel inside a Glad sandwich container; it's not perfect but it is simple - just don't let the damp paper towel touch the baccy)