My hubby and I don't usually flush unless there are solids or a smell is developing. Our well tends to pull up grit in the summer, but even if it didn't, I know that water is becoming a huge concern in many areas around the world, and waste is never a good thing.
At other people's homes, I sometimes ask what their preference is.
Singing: "If it's yellow, let it mellow; if it's brown, flush it down…"
Tracy's got some good tips, too. At our house, we actually don't wash the bath towels until they ask for it (begin to smell unfresh), and hang them to dry quickly between uses, so they last for at least a couple of weeks. We have yet to catch any dread diseases or disfiguring rashes from doing this. I have some towels that are nearly 40 years old and still in good shape. Frequent laundering breaks them down faster, and uses HUGE amounts of water.