when i had rays and was feeding blood and black worms heavily, i wouldent bother syphoning any out. when i would do a weekly syphon, i never noticed a large ammount of uneaten worms.
most fish are going to go crazy over live worms, and wont stop picking though your substrate until almost every last one is gone.
the day after you feed them, syphon the water and see whats left in the bottom of a bucket. if its alot of worms, either cut down your feedings, or start syphoning out a few hours/day after feeding them. if you find none, or not many at all, i wouldent worry