Do you really have accuracy problems with a worm drive? My rotary axis is based on a standard dividing head with the worm driven by a stepper and is plenty accurate enough for most purposes in metal. Just what accuracy are you looking for in the materials you are using?

The only thing a harmonic drive gives you over a worm is zero backlash. If your accuracy problems are due to backlash then adopting the right cutting strategy can deal with them - basically make all your rotary cutting moves in the same direction so the backlash is "taken out". For example I have written a knurling wizard in which the whole knurl is made in one long cut with the rotation in the same direction while the work moves axially backwards and forwards relative to the cutter.

As for the ratios there is the usual tradeoff between resolution and speed. Assuming a 200 step motor with say 8x microstepping and a 10:1 ratio you will need 16000 steps per rev which is probably enough resolution for anyone! 0.0225 degrees per step.