Those of you who followed the other thread concerning the Etiquette quiz may recall that the answer to the question about leaving the toilet lid up or down was that the correct etiquette was to leave the lid down.
Personally I would have thought it was to leave the lid up - simply because the next poor soul to use the facility may be in a rush and not have enough time to prop it up themselves, particularly if they are suffering from something like gastroenteritis.
Having said that, I like to leave it down so that when the toilet is flushed, it doesn't spray bacteria all over the room. When I was studying biology at university we did a study on this. If you place marked bacteria in a toilet, then flush it, you can subsequently detect the marked bacteria all over the room. In a typical bathroom, that means on the soap, on the counter, under the shampoo bottles,
on your toothbrush...!
This is also one of the reasons why I prefer the old Australian style of housing (the toilet being located in a small separate room specifically for that purpose) over the American style of housing (toilet located within the bathroom). (Not meaning to digress but this is one of the reasons why there is confusion over the terminology. Americans asking for the bathroom in Australia will often be directed to the wrong room

).