There are two really good reasons why you should always close the lid of your toilet seat.
1. Closing the lid every time will prevent water from evaporating in your toilet.
2. Closing the lid before you flush (especially after #2) can prevent the spread of harmful bacteria in the air.
If you flush with your toilet seat up it would be a good idea to keep your toothbrush covered or as far away from the toilet as possible. The germs and bacteria can fly up from the flushing toilet and find its way to your toothbrush...ewww!