I use a steady hand and a small brush. Toothpicks attached to the center hole help me rotate the wheel. Thin you paint about 50%, and let capolary action help.
I've thought about making a template, but each vehicles road wheels are a different size. Besides, unless you make a bunch of templates it's going to get messy.
Masking off? The time it takes to mask them is as long as it takes to paint them by hand.
After a good wash and weathering any small errors will never be seen.
I'm sure there are other ways of doing this but this has always worked best for me.- Mic