The best answer I can give is "I don't know". It is not what you want to hear. I think the polination will be okay but avoid spraying sulfur directly on the flowers. Preventing powdery mildew is important.
I sprayed Neem oil on my Grenache during flowering and have lots of shatter. I’ve been unable to find information on whether the spraying might have caused the shatter, but it seems logical. If rain during flowering can cause shatter, wetting with the spray probably can, also. Any thoughts?
My opinion is...no spray on the flowers at all, if you can keep from it. BUT I would not be adverse to spraying the main trunks of the vines and the ground beneath the vines. ( sulphur ) Anything you can do without spraying the grape flowers would be a good thing. After they are pollinated and start making baby grapes, get back on your regular spray schedule/method.