It's actually quite a pain if you don't live in Dallas proper. The viewings are Mon-Friday before the auction. With work and all... Well hard to justify taking a day off to go see a poster. But I was able to view two of the posters I wanted the morning of, Saturday. It would be super cool if they kept the viewings right up until the auction started on Saturday.
That seems to make the most sense to me, too, for those that might want to go and not only preview on the Sat of the auction, but then stay and watch. Ive been to a number of (non poster) auctions here in LA (fine art and works on paper), and everything was available to view, right up until about 30-40 min before the auction started, so they could slightly reorganize, in prep for the auction's start.