Oregon specs filing their chisel chains at a 10 degree 'file guide angle' in their
Maintenance and Safety Manual but then states,
"
When using an Oregon Filing Guide, ALWAYS hold it level, 0 degrees, flat on the top plate" (their emphasis).
I believe that this is because those, and similar file guides, are designed to ride on the top plates and depth gauges of the cutters. This is so the file is held at the right height to form the correct hook shape on the cutters. If you hold these file guides at an upward angle, the file does not contact the cutters correctly.
OREGON Maintenance Manual
But, to confuse things, their illustration describing 'file guide angle' appears to show their filing guide being held at both 0 (or 90) degrees and at 10 degrees!!!
To me, this suggests that a 10 degree angle would improve performance with chisel chain, but is not required. Similarly, some of the Oregon/Tecomec style grinders have a vise tilt feature to allow this angle, and some of the more basic models do not. Tilting the vise for each side takes a few extra steps; more if you go back and forth.
What I have found from sharpening other people's chains is that it is best to pick one and stick with it, whether filing or grinding. If I grind a chain at 0 degrees that was previously sharpened at 10 degrees, or vice versa, I have to remove more metal for the wheel to make full contact with the cutter. This means that I am shortening the life of the chain by going back and forth.
Philbert