Going early definately increases your chances of finding that one of a kind unique piece and potential terrific deals, but there are not a ton of "bargains" period. I have made several purchases over a three day period at several Nationals, so don't feel like you can't find anything unless you go early. There is plenty of material, and many of the purchases on setup day reappear in other booths as the show moves on.
Go, enjoy yourself, and make a couple additions to your collection.