Do you wash ever piece of clothing after wearing it once?

Most pants (in the American sense, not British) should last you at least a couple of days. I'm not going to wash a nice pair of woolen trousers every day.

Not washing them is actually pretty reasonable. I wash my raw denim jeans roughly every 4 or 5 months; this does coincide with how often they actually get dirty.

Nope. I was them when they get dirty, just like anything else.