I've found the strongest vocalists don't really care too much if they hear verb or not - but then again, they already have plenty of confidence in their ability. Although, I do find most vocalists do like to hear at least a little verb.
I do think when recording a vocalist who is not strong and confident, a small amount of verb (not enough to cause pitch concerns) can be a good thing. I do agree that keeping he vocals low in the headphone mix is also a good thing. If a singer lacking in confidence hears too much vocal it can be intimidating which simply results in the singer pulling back (normally not a good thing)- and as already indicated, keeping the vocals low in the mix can entice a singer to belt out a little more which in most musical genres is preferable.
However, at the end of the day I'll do whatever the vocalist may ask for (within moral, ethical limits) to help him/her provide the best performance possible.