In reply to Julie Black:
almost everyone does this if they need to whether they realise it or not. If your on single pitch stuff you've normally got enough rope to lower off without running out of rope. If your abseiling, most people will tie a stopper knot in the ends or at least watch the ends hit the ground, and if multipitching you'll be tied in to either end anyway, doing away with the need for a stopper knot. Fishermans is a better knot though
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