If it's long enough for another pen, then yeah, I save it. That makes it easy when some says "I want one that looks like this other pen you made."
If the blank is too short to make two pens, I take the cutoff and make a keychain from it. Quick, simple and cheap. Plus the length isn't critical. I can then sell them as a set or separately.
I also do this when I am making a double barreled pen and have a blow-out on one of the halves. I just turn the other half into a keychain. Works well with 7mm and 10mm pens. Not so good on capped pens.