Personal preference. Tins kept in a sensible spot should keep their seals for decades. Sometimes, they don't. So some people choose to transfer to mason jars or use vacuum seal as insurance.
If you are going to smoke it within a couple weeks it’s fine in the tin. Amy more than a couple weeks jar it upon opening. Truth.
If I’m going to store it unopened I usually seal in a food saver bag but haven’t had any issues storing unopened tins for several months with the intent to smoke soon.
Oh now i understand you mean a tin you have open and are smoking through.
Still personal preference. Most tins come with more moisture than you want for smoking, so it does not matter if it dries out a little over the week/month of smoking.
Once I open a tin for smoking, the contents go into a latch-top jar. Since I keep several tobaccos “open” at a time, jarring a tin like this guarantees that it will still be properly humidified whenever I come back to it, which may be weeks or even months.
When I open a new tin, I dump it into a jar right away. I keep the tin, and I’ll pull a pinch or two from the jar and put it in the tin. That way, I’m only drying enough for a few smokes at a time rather than drying the whole batch at once.
I don't jar any of my tobaccos for long term storage.
When I crack a tin I transfer it to my bedside container for immediate consumption.
I keep about seven different blends open a time and when I exhaust the contents of one I grab another from my cellar.
All my tobaccos are stored in their original tins. No problem so far.
I typically take two or three weeks to smoke through a tin and typically the tobacco is fine over that time frame. Of course, I like my tobacco fairly dry.