All of what I do is for small instrumentation where fine finishes are the rule.
For steel: I keep a 46 white aluminum oxide wheel on the machine most of the time. IMO, the main reason for going to a finer wheel isn't finish, but the ability to hold a small radius in a corner. I've never found much advantage to a finer wheel than a 60, and they tend to burn easily. I have some finer wheels and they're used strictly for form grinding. Many/most people overdress. I use a single point diamond, take off about 0.0005-0.001", and crank that sucker across about as fast as I can. You don't want to go slow and you never want to make an extra pass with no feed. I remove about 0.0002-0.0007" of stock per pass.
For aluminum: I do exactly the same thing with the same wheel! The only difference is I spray the surface with WD-40 between each pass and maybe remove a bit less stock per pass than I would for steel. No doubt one could optimize the process with a different wheel, but that's too much messing about for one-offs. I've found no problem going from steel to aluminum or vice versa.
Avoid fancy expensive blue seeded wheels for small grinders as they don't perform well. Grinding conditions are just wrong for them.