I went through mine several years ago, and weeded out a lot. I kept cards from my brother, grandparents, and the very few letters and cards to me from just my dad, since they're all gone now. I've also kept some letters and cards etc from my mom, when I was in college, as well as some that she gave me that I sent to her and my grandmother. And some other random letters that meant something to me, that I received over the years. I bought one of those pretty accordian folder things and keep them in there. They really don't take up much space either. But other cards and letters, birthday etc? I usually keep and toss eventually.
I also have a second folder with cards and notes I received from friends, coworkers etc. when my Dad passed away. I don't know that I will ever read them again, as i can still recall, sitting on my bed, reading them all, and bawling my eyes out (I had been gone for 2 weeks so there were quite a few waiting for me) but i can't bring myself to let them go.