I use my scraps continuously ... when I finish a project I trim and sew the trimmings into a larger single piece. That way I am more inclined to use it. When I did not have much storage space I would use the super sized zip loc bags for storing the pieces, compressing the air out as I sealed. I know some use the space bags , that you vacuum the air out.
As for how to purchase. .. if you make a lot of quilts having a roll is quite handy. If you don't produce too many buy it by the yard when its on sale. Maybe buy enough for three quilts at a time.
From what you said it sounds like you need to make use of your trimmings before you accumulate so many. I like to keep my battings to one or two types , this makes the reuse of trimmings much easier.