Personally I won't go to a walk-on or punter day (say for a work trip) without a priest. Blessing the paint with holy water substantially improves accuracy.
Mind you only do it in small batches though, as you obviously don't want the paint to get too wet/swollen. Obviously it costs more to retain a priest's services for the whole day (do offer to cover his entry if he'd like to play as well) but if you are in a big group it's generally worth it.