How a person handles a toilet seat isn't just a matter of decorum, it's a window into the human soul. The type of fella who leaves the seat up after he drains the main vein is the type of fella who leaves clean clothes in the dryer and picks them out when he needs them or wears a Bluetooth device when he's not using it. In other words, he's not to be trusted.
Good seat work is a two-step process. If you're standing up while using a toilet, lift the seat before you unleash the stream and put the seat back down when you're finished. This technique serves dual purposes: It keeps you from accidentally spraying the seat, and it protects the next user from falling in.
As with most rules, there are some exceptions. Situations in which a toilet will be used exclusively by men call for a change of plans. Rather than going through the motions of lifting, dropping and lifting the seat each time the john gets used – and taking the chance that one of your more lazy friends, colleagues or roommates will forget to lift and miss his mark – go ahead and leave the seat up. Just don't get used to it.