Jezebel had a great article a while back covering bathroom etiquette, which boiled down to "Leave the bathroom the way you found it".
I think there's a difference between a public, private and shared bathroom.
In a public washroom the facilities are managed by paid staff that hopefully check the room regularly and clean up any messes. No or very little cleaning supplies are available to the average user.
In a private washroom, the facilities are maintained by yourself or your hosts. I'd be mortified to leave a mark behind in my host's toilet and would use whatever was available to clean it up. Likewise, if a guest left a "surprise" for me I'd probably think twice before inviting extra work again.
In a shared washroom, such as an office or a roommate situation, I think people should behave as much like it was a "hosted" washroom as possible. Keep all traces to a minimum at all times. There should be supplies to clean up if needed. Each stall in our shared ladies at work has a toilet brush. That's so you don't have to leave the stall to clean up the low-flow toilet if necessary. I usually give the seat a good wipe and drop the tp in the bowl before starting, ounce of prevention and all that. "If you know you have to go, grab a sheet and lay a row." Or something. I think that was for "unladylike noises".
Another thing the article touched on was little bits of tp on the floor. Not only is it gross to litter, small, white, slippery pieces of paper on slick tile or marble floors can be dangerous. Just pick it up! You're washing your hands anyway, right?
