I don't think the timing will matter that much for you, since you're not going to the first day of the expo. I am generally going to the expo on the first day it's open and have had bad luck with lines going in the morning, not necessarily for W&D, but I think it holds true in general across the race weekends. What I've found works best for me is to go to a park in the morning and head to the expo early to mid-afternoon. The lines and congestion seem to have died down by that point and it's usually a very quick trip. I don't really care about merchandise, though, so if you're looking for specific items that may change your plans/priorities.