I reuse the soil for potting up plants. :-) I actually reuse all my soil.

Yes and soon.

I'm a super-thrifty wintersower, and not only do I reuse mix from dud jugs, when I plant out I knock as much of my potting mix off as I can--barring true root injury--and put it back into "stock."

You may get your dud seeds sprouting after another winter or two--that's happened to me once that I know of--but you could scrape the top inch of mix off into a special spot and reuse the rest. That way if something comes up there, you know it may be a keeper, not a weed.

I use the old soil to pot up new plants for swaps and giveaways, but I don't use it to start seeds. I also cut off the tops of the milk jugs and use the bottoms for potting up so I'm not constantly scrounging for pots. I go to a lot of plant swaps and give away lots of plants every year and this saves quite a bit of time and money.