With regard to precutting the teeth (called gashing), I think it would be necessary if you wish to control the tooth number. Either that, or you must have a positive drive of the blank in the proper ratio to generate the proper number of teeth.
The explanation is that the OD of a gear has enough extra circumferential length to add in two more teeth. This is the rule for calculating the OD of a gear blank: add two to the required tooth number, divide by the diametral pitch and you get the blank OD. Actually a standard wormwheel is shaped with a bit of a hollow around the OD to improve the wrap of the wormwheel tooth around the worm, so the OD of such a worm wheel is calculated as the (required tooth number +3)/diametral pitch.
By the time that the cutter has descended to full depth, then it should cut exactly the required number of teeth on the pitch diameter circle. So where and how do those 2 extra teeth have to disappear to?
So if you start out with your hob gashing the smooth blank, as it progresses to full tooth depth, the hob must tolerate this mismatch between the pitch diameter and the number of teeth it started with.
If you are clever, you might be able to figure a way to position your hob at full depth to begin with, and feed it tangentially into the blank, but most likely you will get some slipping of the blank going on, and it may spoil it unless you have a positive drive of the blank in time with the hob rotation.