A perfect right angle cut steerer is not essential but that is what I aim for to the extent that it is possible. I hit 50.3 MPH on my road bike this weekend. The correct cutting tool and a $15 guide doesn't seem like much to me.

Probably more important than a perfect right angle is to cut it smoothly so that stress risers don't develop (steel and aluminum) and to ensure that carbon doesn't splinter and start cracking. Smoothing out burrs and rough edges also helps.