ALL TIPS lose their perfect shape, whether dime or nickel, after a few racks. It's just the nature of leather. The center dome of a freshly shaped tip will flatten some in no time, regardless if it's a soft, medium or hard leather. There's not a lot of leather at that dome and the shape of a newly shaped tip is just a starting point for the eventual shape that the tip will take while playing.
If you keep shaping the tip to keep it the shape of a dime, it will wear out in no time.
This......