Depends on the show. The software I use, and most judges from what I understand, want to do a 'bump' or 'advancement' method -- a fish gets 'best in size' before being in running for '... champion' and a new 'best in size' is chosen.
I think with smaller shows and some judges the preference is pure merit. If some fish just aren't good enough for a first place, then there is no first place in some groups. Ditto for bumps to best in size, but I'm guessing most shows have enough quality fish where it is only the 'ribbon' awards where it matters.