Facing the shell won't make a significant difference to the head angle. Headsets often differ in their lower stack height by quite a bit anyway. A 10mm difference (which is a lot) might be half a degree or so, which I know I wouldn't notice in the handling although some people might be better at detecting that kind of thing.

I always use transmission fluid as cutting oil as I happen to have a bottle of it from when I thought I had a power steering fluid leak one time which it turned out I didn't. I think anything oily is fine.