Finish works if you want, just don't expect them to be good.
I'd say just doing exercises for a while is probably a more productive use of your time while you're getting the basics down, but getting into the habit of actually finishing things is good, and you can learn plenty from doing something from start to finish and trying to refine it. A lot of people compulsively start things over and it stops them from working through their mistakes, they just restart with the first fuckup and don't learn from it. If you're trying to finish something, it means you're trying to identify and fix what you've done wrong, and that's vital for learning.
But still, know when to cut your losses. Don't get too attached to your stuff and be ready to scrap them if you're just polishing a turd. Know what your goals are when you start a piece, and if that goal is "have a polished, finished piece" then finish and polish the fucker. But if it's "learn x", you don't need to follow through and polish it if you can learn x more productively.