Photoman: All kidding (and banning anti-smoking dudes) aside...
Some tobacco's have some anti-fungal in minimal quantities. Some will have Propylene Glycol to preserve moisture. It is more dependent on the blend and blender than it is on whether it's tinned or bulk.
I think Sutliff Reserve tobacco's, which come in a tin, may have some PG added. Some bulks, such a Kendal's Kentucky, does not.
Some of the articles Kevin referred to in this and the other post may help. Personally, as long as it's not a goopy mess (which doesn't mean PG but could be just too much casing or topping), I really don't worry about it too much. As a 35+ year nail smoker, I'm convinced pipe tobacco contains less crap than cigarettes do.
EDIT: And I found this quote out there on Dan Tobacco, but a different blend
The cut is very fine ribbons which makes for easy loading and minimal relights. No propylene glycol, no goop. The taste is simply Virginia tobacco and vanilla, no more nor less.
"The fact is, squire, the moment a man takes to a pipe, he becomes a philosopher. It's the poor man's friend; it calms the mind, soothes the temper, and makes a man patient under difficulties. It has made more good men, good husbands, kind masters, indulgent fathers, than any other blessed thing on this universal earth."
-"Sam Slick, the clockmaker" aka T.C.Haliburton