I have been travelling for years with opened tins in my check-in luggage. I don't even put them in baggies and usually wrap them in spare beach towels that I always carry for this. Nothing weird has ever happened. You are carrying (for now, anyway), a legal substance and if a TSA agent does happen to open your luggage and twist open an unsealed tin, just hope he twists it shut again after examining the contents.
If you carry sealed or unsealed tins in your carry on luggage with your pipes, expect to be pulled out of the TSA line and have them go over the pipes and run some kind of chemical wand over the tins. I've had this happen half a dozen or so times as pipes can look like small pistols on the screen according to TSA. Once they see the tobacco, naturally they want to have a look at that, too. I still put my pipes in carry on luggage, but not tobacco.
I have a theory - no proof - but I think the reason why (particularly rectangle factory sealed tins) sometimes arrive unsealed after air transport is because checked luggage is kept in a non-pressurized environment during flight. The changes in air pressure (I believe), is what causes the tins to lose their vacuum. And of course, the older the tin the bigger the chance. I can't count the times over the years that I've had old factory vacuum sealed tins shipped to me for consignment sales and when I open the boxes, I find an old tin that has lost its seal but the tobacco is still moist, an indicator that the tin was shipped still factory sealed but didn't hold up to the air pressure changes.
