Similar to Lance, I don't use preventative fungicide for deflasking any more, neither. I do sometime use Aspirin water 1x or 2x to boost the immune system a touch. Since they were in sterile environment, it may be good to give a bit of challenge against the immune system. I wouldn't over-do this, though, and this could be just a theoretical practice since I haven't done a controlled experiment yet.
I had to use Physan-20 (1tsp/gallon) recently on seedlings 2-6 weeks out of flasks (P. mastersianum from Rick's cross, P. vinifierum, and 2 Dracula species). It was related to bush snail prevention, and none of them were affected since the plants were very healthy out of flasks. But I would rather avoid the possible risk of phytotoxicity if I had a choice.