Well, what I'm about to say may have already been said, but at the risk of being redundant, I'm going to throw in my 2 cents.
There are only 2 reasons I would toss a spool of thread. If I am sewing and my thread starts breaking, or acting up, I remove my thread and take a good look at it.
If it appears to be uneven on the spool, not smooth but bumpy or crimpy, I toss it. This can occur any time on a spool, it happens during the winding process. The crimps are permanent, once they are there that's it.
If the spool looks okay, I let about 8" of thread drop from the spool. As I hold the spool up, if the thread curls or loops as it hangs, it will curl and loop in my machine as well. It should hang perfectly straight. I'll toss it if it doesn't.
If you have metallic thread that causes you grief, check how it hangs from the spool. Not straight? That's why you are having problems with it.
If you wind a bobbin with thread that was uneven on the spool or does not hang right, you will experience problems when you sew.