Great question, and I love the word "sackslip."

At a reeeeal and proper yoga studio nobody cares; nobody is looking at you anyway. (This is why e.g. naked yoga is sort of a thing; it's an internal practice — the world ends at the edge of my mat)

In real life, there's some basic courtesy involved. I'd say go with something that keeps your sack from flapping about + extra separating layer between skin and polyester (do NOT wanna be sweating in polyester, esp not on balls)