Yep, I used the sharp scissor method, too. They were cheap onesies I had with DD#1, and were starting to fall apart anyway, so I cut off the bottom. Not terribly cute, but functional and great when it's hot. I just washed the ones I cut, and no fraying. It's definitely the way to go.
Re: has anyone converted onesies to tshirts?
well, if by converting you mean slicing off the bottom with some good scissors, why yes, yes i have
i'm sure it would be easy to hem them too, but i despise hemming. and they were cheap ON white onesies anyway.