I very rarely remove a set trigger from its bed in the stock. On the occasions when I've thought it necessary, on my own rifles, I have found the triggers to be in perfect condition without any rust, but I cannot say the same for some clients who bring theirs in to the shop for maintenance. I find dried up oil and dirt, rust, bits of leaves and twigs, etc. Refurbishing them is easy though...I find some locks in poor repair too when they come to me. On percussion rifles, I often find a build up of fouling caked on the underside of the neck of the hammer, and in the cup, and I can almost guarantee that the bore is not pristine on those rifles. If the shooter can't clean away fouling they can see, what chance is there that they get the fouling they cannot see.
Like Daryl, I remove my barrels to clean as well. I use finishing nails that measure .072" in diameter, and I face the ends and drill a tiny indent in the centre on the lath with a 1/16" centre drill. Then a punch with a corresponding tit on the tip fits the pin perfectly and is undersized, so it pushed the pin out enough to pull with pliers without damaging the surrounding wood in any way.
I also fill the pin's holes where they are recessed beneath the surface with darkened beeswax...keeps water out and I don't like seeing shiny pin ends along the side of the rifle. Just personal phobias.