I doubt they are as concerned with overcharge as they are with stealing. However, it does help consumer as well as the store.
I shop at BJ's which is the same a Costco(we have both here), it's not too big an inconvenience to show your receipt. I have more issue with people asking for email, address or zip code, nobody needs my personal information.
You really have to check the prices at these wholesale stores. I've found my supermarket has better prices in most instances. It should actually be illegal to state that you are a wholesaler and then sell products at retail prices.
Here's what I recommend:
Bottled Water: Good discount here.
Facial Tissue: Mild discount, but the best you may find
Shaving Razors: The pack bundle is a better deal
Books: This is borderline, because you can go to a used book store.
Propane: Best deal you can get(and, make sure they weigh the can)
Other than these, look hard at price consistency as it is all over the map.