They have computer algorithms that are fairly accurate in predicting the number of no-shows, so it probably doesn't bother them much.

I try to cancel if I can just so there is minimum hassle for the GA at boarding time; it just seems the civil thing to do. I don't get too worried if I can't contact them in time, either.

If your credit will be >$5 after the change fee, why not cancel and at least have the possibility of using the credit later, though?