Yes, it is a good idea to always ask first because the stores are having such a hard time staying in business. Some stores may allow you to take the picture if you are convincing about your motives.
On the other hand, some of these store owners are downright paranoid. I don't like to shop in a store that aggressively assumes that I am out to cheat them. I know of a store in Colorado who won't allow coats in their store in the winter because some people steal items and hide them within their coats. That's silly... theft is part of owning any retail establishment and has to be part of the costs of ownership. This means higher prices for their merchandise which leads to people shopping online. It's sad...