I save everything scrap and even the threads left over. I do try to sort them by size but there are times I can't be bothered and just throw them into a trash bag and every couple months go through them and just take them randomly and start sewing them together. No rhyme or reason for it. So far I have amazed my friends and other quilters in two guilds I belong to and really amaze myself at how things turn out. I get remarks from some of them that they wish they could do that and I tell them they can. Just take pieces and start sewing. All of them are "cookie cutters" as I like to call it...have to have a pattern and cut it to be able to start sewing it.
I'm very much the opposite. I'd rather just take different fabrics and just sew and see what comes out of it.
Today instead of sewing at our Wednesday sew I took a full trash bag full of left overs strings, and fabrics and started soring out and yes even selvages and placing them in smaller plastic bags so I can see what I have. One lady told me that was all trash and it should go out in the trash cause that is what she does. I suggested that perhaps she should find someone to give them to like me and would make good use of it. She said she'd think about it but I seriously doubt she will. I know another lady that throws away all left over pieces of fabrics instead of donating it to groups that could make quilts with them to give to charities. It is her right I suppose to do so.
I have plans for what I have even to as little as 1/4" strips.
The left over threads I'll use and blend in needle felting projects or appliquie fabric projects.