For what it’s worth, I always use the tub. I assume the snakes have at some point gotten urates or fecal matter on the bowls and don’t want it in my sinks. If I know the bowls are soiled, the water gets dumped in the toilet, the bowl washed with dish soap in the tub, and the tub gets a quick scrub when I’m done (only if bowl is soiled).
On the rare occasion one of my snakes using their water bowl as a toilet, I dump it down my actual toilet as well. Good point!!