I think the real question is- what do you want to do with those miles? Visit other countries? See family or friends? Try out different airlines and classes? Yes, conventional wisdom dictate that miles have a certain value, but really, the value is based on what your goals and priorities are.

In my case, I'm usually gathering miles for some particular award. Once I reach that amount, I redeem the miles. There's no reason to just sit on a large pile of miles since they don't gain interest and program rules can change at any time.