Reuse them. I use paint strainer bags or bags sewn from voile. The hops stain the bag, the grains not so much. Staining is not a sanitary problem.
I rise off any residue then turn it inside-out and repeat until almost all the little bits are gone. Almost impossible to get every last grain fragment, but two or three bits have not hurt anything. Squish them in a sanitizing solution. Hang out to dry like DW's nylons. Just before using, rinse and pass through sanitizing solution.
Note on sour beer: another thread suggested separating the plastic equipment used for sour beer from that used for regular beer. The reasoning from experience is the bacteria necessary for sour beer hang around in the scratches of plastic fermenters and the weave of plastic bags.