I did it because DD would only sleep in the crib if it was slightly elevated. We took a pillow and put it underneath the crib so one end was slightly higher and she has been sleeping in her crib since. I am not sure about the spit up situation though.
We did to help with spit up. I don't know how much it helped, but we folded a towel and put it under one end of the mattress. We kept it there until she was about 2-2.5 mo.
We did because of reflux. We just put books under one side (so 3 inches or less). I think it helps. You can also get a thing to go under your mattress at babies r us for about $14.
Re: Elevating head of crib