I always put my initials and the date on a piece I give away. However, I have been thinking on really big/complicated works, I need to put a tag on the back giving my full name and date. This is great for historical purposes, when you hand something down through your family. This is done for quilts.
I always initial & date with the last 2 digits of the year in small font at a suitable place on my work (normally a bottom corner somewhere but it does depend). Just like signing an artwork - shows who did it & it adds a personal touch.
I always backstitch my initials and four digit year. I havent gotten any pieces framed yet, but when I do I think I will put a note on the back with more info.