definitely make before you start stocking up on them. You might find you use them less often than you thought. I almost always come up with other ways to drive stuff, like holding one end in a 4 jaw chuck. If it's worth setting up with a dog so I can remove and replace a couple of times, then it's probably worth my time to make a quick dog for it.
I often find the pre-built dogs don't fit in the setups I want to use them in anyway. Like the bent one doesn't fit when I want to use my chuck, and turn a center in a bit of stock held in the chuck. I wanted a dog to place against the chuck jaw, but had to make a shorter one to fit.