The LH/RH in that respect is to do with the physical position of the cutting edge, not the type of thread.

There are holders/inserts available for turning that require angled shims, to ensure the insert is cutting at the correct angle for the required diameter, and ensure the correct thread form is produced. Larger diameters for a given pitch, have a less angled thread, so to ensure a 100% correct thread form, the cutter has to be angled to compensate for the thread angle, however if you need that level of accuracy in your threads, you probably won't even be considering thread milling.