I've always used a Dremel to cut housing (in fact I'm still using the same one I got for my 14th birthday); I've never worried too much about the less-than-a-millimeter off-angle cut, it doesn't seem to affect anything. I guess if I absolutely had to be precise, I'd rotate the housing while cutting it so that the angle remains consistent throughout the process, then smooth it out and ream it with, respectively, cross-cut & rat-tail jeweler's files.
Aside from wearing safety glasses, a bit of advice- when you cut, use the highest speed setting and cut with the top edge of the cutting wheel, not the bottom (so that the sparks & shavings are being spun AWAY from your eyes & clothing), and cut slowly. Cutting too fast torques the cutting wheel and can cause it to shatter; cutting too slowly however, will melt the housing. The cutting wheels are actually kind of fragile when it comes to lateral force. They'll also shatter when you wear them down too far.