I've struggled with the same issue, thinking it is more efficient and comfortable for the patient to put the block in while asleep.  After considerable reflection and discussion with my peers, I have gone back to doing the block while the patient is awake.  Even if you see contractions with the nerve stimulator disappear before you hit 0 milliamperes you can not be assured that your needle is not intraneural.  One quadriceps palsy could ruin your whole day   : >(