I really hate buying something at one store only to find later that I could have gotten it cheaper at another one. I'm currently in the process of coming up with a database that will tell me the different prices of different things at different places. Unfortunately, it's a work in progress as I'm not about to go through three different stores and take notes on every price. Since we're just in the early phases of cooking, we don't even know what our staples are going to be.
I found out today that one of the local grocery stores has a site that shows their weekly ad prices. After working in retail for many years, the general rule seems to be that big grocery chains generally have higher prices than say WalMart or Aldi. But they also tend to have the best sales. Checking sales online before going shopping can save gas money and a small piece of mind.
In this case, I made a list of all the sale items that would work with this coming week's menu and their sale prices next to them. I'll take this list when I go to one of the other stores and will be able to compare sale prices with regular prices. Granted, this doesn't work on regularly priced items, but it's one step to take to make sure you're getting the lowest possible price on groceries.