With library books, I think the concern is more about wear-and-tear on shared property. Some of us leakily generalize this to “folding page corners is bad”, even for non-shared books. When it’s your own book, you can do whatever you want.
Personally I find folded page corners less effective than bookmarks for quickly finding my place, especially if I’ve folded many other page corners, which makes the currently-folded one less visually obvious. But perhaps I’d learn to be better at that if I used it regularly.
With library books, I think the concern is more about wear-and-tear on shared property. Some of us leakily generalize this to “folding page corners is bad”, even for non-shared books. When it’s your own book, you can do whatever you want.
Personally I find folded page corners less effective than bookmarks for quickly finding my place, especially if I’ve folded many other page corners, which makes the currently-folded one less visually obvious. But perhaps I’d learn to be better at that if I used it regularly.