I usually try to use up whatever leftover currency I have unless I know it's some place where I'm highly likely to be back within a year (JPY, EUR, GBP, SGD). That also guides my ATM/exchanging habit as I don't want to "have to" exchange things back or spend at the airport.

A couple of times, I've had more of something than I planned on using in the near future and figured I'd just exchange it for something on the next trip versus exchanging back into USD.