Not at all! At least you'll know you'll be getting something they want. My partner and I ask each other every year. It doesn't ruin it. We usually start asking months before Christmas or our B-days and then keep it a secret that way it's still a surprise. We actually go to great lengths to keep it secret. I had a real chaotic time shoving him in wal mart while I drove to Game Stop a few weeks ago. Even with asking him what he wanted, he still had no clue. Just tell them to give you a few things they want so that way they don't know for sure which you'll choose to give.
Works for us. He got me a Kindle Fire and couldn't wait so gave it to me early this year, but he did surprise the heck outta me on Christmas day with some zebra print heels!
I tend to think it's better to just ask.