As a sub, I do all three.

I might cry at the beginning from fear, frustration or vanilla-to-sub breakthrough.
In the middle, from pain, intensity, deep submission.
At the end, to relieve profound emotions or to express submissive joy. I prefer this one!

None are a reason, in themselves, to stop a scene. Just work out why I'm crying (might be the desired effect) and deal with it appropriately. Like many females I often find tears cathartic and cleansing.