If you do it in one pass with a hand turner then the first cut might have some spring in it so the cutter will also cut on the way back off the neck.
If this is the case then go slowly on the way back to allow a smooth cut.
This is in effect a second pass anyway.
I prefer two cuts so the last cut to size is a smaller one having little spring.
It all depends on how much you are removing , how sharp the cutter is , how rigid the turner is , how good the mandrel fit is , how fast you go etc.
Its the finnished product that counts and as long as you are getting a good smooth parrallel job to correct size then that should work good.