To be honest, tyre wear is never fully even, so even if you could round it off again it would not wear perfectly anyway. Also, if it is squared off, it has worn through a lot of the rubber that gave it its original shape, so you would just be wearing more rubber off.
I tend to ride my tyres until I have to replace them because I am skint, but if you can afford new boots then do so. I replaced my last set with Continental Contimotions and have not looked back. They are grippy in every condition I have ridden in (although the rear span up nicely on the wet grass in my Aunts Garden!
), have a responsive but not nervous feel to them and feel good even when loaded up with a pillion and full panniers.