I try to get all of them labeled--although right now I have 4 setting upstairs for a guild show and none of labels! I usually put:
name of recipient
my name as maker & sometimes my town/state
date made
occasionally I'll put something additional. I've printed from my computer and sewn on, hand written (saw something about that being a ice touch for future generations)and sewn on, embroidered by hand right onto the back or back of binding and when I give a t-shirt quilt to nieces and nephews to take to college, I usually try to also put their name and hometown in the quilting just in case someone steals it in a group living arrangement.