Ha, opposite to Mary, I tend to size up on bottom but not on top because I am huge on the bottom.
In all seriousness though, I really have to go back and forth on this. This last two jeans purchases I made, I really could have gone a size down -- both are at Nordstrom being taken in at least two inches in the waist. I doubt the next size down really would have been that much tighter on the thigh. Oh well.
Tops are a little easier for me, but still tricky. Sometimes I have to size up to accommodate the bust even though the rest of the body is big. That sometimes bumps the sizing up to where the shoulder seams fall too wide, which means the cut is just plain not for me. I just took two tops to consignment that fit everywhere beautifully but pulled ever so slightly on the bust, so in retrospect, I should have tried sizing up.
I'll be honest, I kind of hate sizing up. The numbers bum me out, as much as I know they're meaningless. Especially if it involves a jump to a double digit number. I know it's awfully vain and silly, but that phenomenon has been to blame for a few purchasing mistakes over the years.