While it has been mentioned, I want to repeat and emphasize it because it's the main point. What beginners do, after all, is only relevant for beginners, but I

While it has been mentioned, I want to repeat and emphasize it because it's the main point. What beginners do, after all, is only relevant for beginners, but I think the question is about violin play in general. The issue is that the locations simply are not fixed. For one, there are different ways to tune a violin, and second, depending on what environment you play with and with what other instruments you adjust your pitch automatically accordingly. Since it is so very easy for the violin this is the instrument which adjusts to the others, the piano can't do that on the fly. I also can confirm that it is much easier than it looks at first. Somebody said most beginners use markings, but the few other beginners I know, but also the violin teachers I know, don't use them (when I say teachers I mean for their students of course). I didn't think about it much when I started a few years ago (at well after 40, but it's fun and works out quite well), I never felt a need for it. What you notice when you play is that it actually is pretty easy to adjust on the fly. You develop an ear for the right pitch (if you don't you may have a problem). In my experience you quickly place the finger in at least almost the right place, I doubt that any marking would help do it better when you are in the middle of a song, unless it's the slowest of slow. For the tiny differences that you need to adjust for markings are of no help because I would say you cannot hit them as accurately as would be needed anyway. Even if I had markings, they would be useless. I would not look at them most of the time, besides, you are looking at quite an acute angle since you are holding the violin with your chin, so you would have to mentally adjust for that too! I think it may well end up being more of a hindrance than an aid.