Decorate it just like the front. All branches are created equal.
Leave it bare. No one's gonna see it anyway.
Decorate it, but feel free to skimp on quantity.
Decorate it, but put the crappy ornaments there.
Put the tree in the middle of the room so there is no back side.
Have someone else decorate the tree so you don't have to worry about this question.
I don't have a Christmas tree, but I feel strangely compelled to answer the poll anyway.