I recycle all my mixes (mainly just used for striking roots before planting out). I find that the quality seems to improve over time. Especially if you start with good stuff. I'm always mixing in new ingredients to top it up though. I'm also a big fan of charcoal (straight outta the bonfire pit), I've heard that charcoal keeps things like salts and PH under control. So far I've never had a problem with any fungus either.

It seems that the non-organic material stays as the same while the organic component only improves with age. Like a good wine I guess...

Call me wasteful, but I wouldn't want to recycle too much potting mix. Certainly I wouldn't recycle seed raising mix without nuking it in the microwave to sterilise it first...

However, if you're constantly topping up exhausted nutrients with fresh goodies, I can see how this would be beneficial for mature plants.

I sift my soil to get the sand back and use that for cuttings.