Hi ,
I always laugh when a machine knitter tells me they save and re-use their waste yarn.

Those same knitters complain about the size of their stash, but they save a couple of yards of 'perfectly good' waste yarn to re-use it.

Do you feel that it's wrong to toss waste yarn after using it only once?
If so, consider using Cast on Rags.  Having Cast on Rags handy eliminates using waste yarn and can make casting on faster.

They are especially handy when casting on a few stitches along the edge of your knitting. They give us a place to hang weights to make it easier to knit those stitches.