I do hats from the bottom up because I can try on or measure them after a couple inches to make sure they're the right size. I don't use the same needles so my gauge is different. Plus they always seem to stretch a lot more than my sts per inch indicate. When I crocheted hats, I always did them from the top down, somehow guessing (or sort of trying on) when to quit increasing to get the right size.
I suppose that in knitting one top down, your gauge would have to be right on in order to make the lower edge fit right. Or you'd decrease before the edging to get the right size.
sue- knitting heretic