I wonder if it's a regional thing. I personally do one or the other, but when I lived in England when I was a kid, it was toast -> butter -> whatever else on bread. Which tasted a little odd to me.
don't know if it was the area I lived in, or the time or what, but all my British friends were totally grossed out and had never heard of peanut butter and jelly. Even accounting for the jelly is actually jam in England thing. (Are we sure that we're all speaking the same language, here?)