I sewed on my Kenmore Elite for nearly 20 years before I realized that I had been inserting the bobbin in with the thread going in the wrong direction. It had it's little glitches but I figured it was just one of those things and for the most part it sewed great. DUH!!!!!!
With the old Coats and Clark thread, with those darn slits cut in the top to hold the thread tales, I always put my top spool on so the thread comes from the back vs. the front, that way the thread never got caught in the slit. Never had a problem.