If you are buying the normal uggs ie: rounded toe go a size smaller. However, the winterized uggs...more narrow toe...better treads on the bottom etc etc go for your actual size. I got a more winter pair this year for xmas in 6 but my normal 7 would have been better. Your best bet is to go try some on to get a feel for what size you will need.
My experience - for whatever its worth lol - is that any shoe/boot that is fur lined typically runs small (the lining takes up space). If it isn't completely lined, then you can probably go your normal size.
But I agree with whoever said you should try them on and figure that out BEFORE you buy.