To American retailers, EVERY holiday is a reason for a sale; shoot, we invent holidays so that we can have sales (Mothers' Day was invented by Hallmark). However, July 4 isn't a 'specially big sale day. You're likely to see summer fun items on sale: coolers, swimsuits, tents, pool toys, picnic foods -- the sales are likely to be limited to these types of things. These same things will be clearanced shortly after July 4 (so the retailers can lay in back-to-school clothes), and the prices will be better then.
I would not make a long trip JUST for July 4 sales; but if I were going to be traveling anyway, I'd make a point to stop and shop.