I've done it for severaol reasons (chronic uncurable finrot/septaecemia, growing back strangely, although that was slightly more than cosmetic, fins have grown in ways that really don't help mobility or betta sanity...). As long as you don't go mad and cut off the whole fin, it grows back fine, and once they're over the initial shock they are pretty much back to normal. Certainly less stressful than having most of it bitten off either by themselves or others.