Late to the conversation, but I always stick to the 500 mile rule, unless I determine the shoes have taken too much.  If I waited for tread wear, I'd probably hurt myself!  Definitely cushioning is my sore spot.  If I feel anything significant or a glimpse of pain that's it.  I separate shoes based on the length of run too.  An older pair may be no trouble for a three mile run, but if I'm out for 10 or more, I want the most cushion possible.  I rotate pairs too so that the cushion has time to "plump" between runs.  That could sound like a big investment, but I'm always searching sales racks and trying new things since every time I find "the one", they stop making it.  The replacement usually is not the same...  Also, as new pairs get worked in, I have a good comparison for what the cushion Should feel like since you never notice the decline.