I learned that stash seems to grow and multiply. My DH suggested that I leave the light on in the sewing room. lol I once pulled out the scrap bin and sewed an entire top from it. When I put the rest back in the bin, I swear it was fuller than before.
As for throwing things away....that is truly everyone's personal business. My take is that I put my "undesirables" into a separate bag and either send it on to someone I know will use it or donate it to the community service group at the guild. It is my "pay it forward" opportunity. It is not really a big effort but it brings a nice reward. (And yes, some pieces end up in the trash at my house too.)