Personally I wouldnt. What if something happened and the model was allergic to something in the viagra? Then youre the one responsible for it. Youre not a doctor and have no business giving it out to "employees" in order for them to perform.
If they want to do it on their own, fine, but I wouldnt supply it for them or suggest they take it before the shoot.

Id tell the models not to cum the night before or the day of the shoot and be ready and hard for filming.

If you do decide to go with viagra anyways, make sure they dont poppers cuz that combination can kill them.