I usually use them for piecing - my machine is drop-in bobbin with a clear lid and clear bobbins, so I can check it and sew right down to the last couple of inches.
But if I need a bobbin, I'll pull off several feet of thread and toss it. Life is too short and I'm too prone to hoarding anyway. I can't save every little scrap of everything! I already save every little fabric and batting scrap to make dog beds, saving every little scrap of thread is one step too far for me.
For awhile I had a thread ball going - I pulled a bunch of rotten but very colorful thread off of about 30 vintage bobbins and wound it all into a ball. I added to it for awhile before I decided I was just being a hoarder and tossed it. I can't say I miss it.
