I usually change the "flavor" with each new bag of food that I buy, not only to give the dogs more variety in taste, but to change up the protein source. You don't have to by any means, it's just something I like to do
My Yorkie is really picky, so for her I've just recently (after much struggle getting her to eat well
) found a brand of canned food that she likes mixed into her puppy dry food and I switch the "flavor" every 2 -3 days and I buy as much of a variety as the brand carries. Every dog is different though, some of them this will upset their tummy, but for mine they do fine. I do stay within the same brand though unless there's a reason for a switch. There are some brands that are actually meant to be rotated through the different flavors, such as Nature's Variety. It's really up to you and your dogs whether or not you do it
although I do think offering a wide variety of protien sources does benefits their health.