Nothing on the outside that suggests what the contents are. Might seem pretty basic, but I once received a Marlin rimfire rifle (very used) in a white Tikka T3 box only sealed with clear packing tape. Got lots of funny looks picking THAT package up, and the lady at the post office now knows that I probably own guns, and knows what my address is.
I live in a very Liberal suburb north of Toronto. Not ideal.
Other than that, common sense rules apply. Anything IN the package that could bust out of it if the box gets squeezed should be packed accordingly. Bolts out of bolt guns, scopes removed and packed separately, lots of padding around the muzzle, etc. I usually make my own boxes out of heavy cardboard but if I happen to have the original box, I put it in one of those...add padding (See above) and wrap the whole affair in another layer of unmarked cardboard. If you saw my EE feedback on the big gun website...you'd see comments about how I pack things.
But a rifle case with any rifle-like shape (in other words, most ATV cases)...I'd say "no" personally.