There has been considerable discussion, along with possibly some truth splattered in, about the quality of merchandise from box stores. It seems true that the manufacturers will package a model a little different, maybe use a plastic pop instead of all metal, to come up with an exclusive model number, so you can't price shop, and also to make a price point demanded by the store and by the consumers.
I have not been convinced that the basis underlying quality of the products is any less. Certainly we know of cases of "bad out of the box" , but this may be due more to the way products are manhandled by customers in the store, and to the return policy.
All in all, I would not discourage you from dealing with a box store, as long as you are willing to accept the possiblity and hassel of getting a bad one and dealing with that. The store of course has a no-questions-asked return, you just have to deal with the hassel. And if you buy from there, you cannot expect a plumber to stand by that product. If a shower valve turns out to be defective, or missing parts, you must pay his labor involved in fixing that. Additionally, if the product later fails during the warranty period, the store, or the manufacturer, will provide replacement parts as necessary, but the plumber will be under no obligation to provide the labor on that repair.
SO, depending on how you wish to deal with it, you can take the discount price, or you can let the supply house and the plumber take a markup on the product, and provide you more personalized service. Your choice.