I never thought about them brushing in a stall and spitting into the toilet. I've never noticed anyone doing that here. For the people who think that's gross, do you mean that, as the brusher, you don't want to look at the toilet enough to spit into it? Because toilets obviously get a lot of gross stuff put in them and then flushed away, so I don't see how it could be gross as in, "that's not a proper use for a toilet."
I'd never, ever thought of brushing in a stall until reading this thread. My thoughts are these:
1. Way more water to flush than to rinse out the sink.
2. Way more chance to get toothpaste spatter all over the place, as you'd have to bend down pretty far to spit directly into the toilet.
3. And therefore, more cleanup needed.
4. No clean water to rinse your mouth with.
5. And it just seems really, really odd to bypass the sink--which is more or less the accepted place to brush teeth, and brush them by the toilet, which is very much *not* a normal place to brush your teeth.
While I admit that it is not the norm to walk into the restroom at work and seeing someone brushing their teeth at the sink, I don't find this activity to be surprise-worthy. I would very definitely be startled if I realized someone was brushing their teeth in the toilet stall, when there was a perfectly good, working sink a few feet away.
From reading this thread, I can see that some people would be bothered, in various ways, by a co-worker brushing their teeth in the shared bathroom at work. My feeling is that this falls under the "excuse me for living" type of thing. The co-workers are doing a normal activity in a place where this activity is commonly performed. If it bothers a co-worker, the co-worker can leave the restroom until the tooth-brusher is finished.
Should the tooth-brushing or flossing be moved to a more public area of the office, then I would think co-workers could comment on the location of the activity.