Well, no, there is no rule that says you can't buy precut “stew” meat at the grocery store. The problem with this is that it’s kind of mystery meat because you really don't know what cut of beef they are using. It’s best to buy a large piece of beef so that you know what cut you are getting. If there is not enough fat or connective tissue, the result will likely be some very tough chunks of meat. If you cut the meat yourself, you can eliminate any hard fat or silver skin and make sure that the cut is against the grain, exposing smaller sections of beef that will be more tender when eaten. Of course, if all you can find is pre-cut “stew” meat, go for it. Trim off any hanging or hard fat and watch it closely as it simmers.