It also wears the rope. This method of adjustment is best kept for peals or other long periods of ringing. If your tower uses ropes with Yorkshire tail ends then you can't do it anyway. Tie it a little way below the sally. Always remove knots after you finish so the rope does not become permanently kinked as the knot gets tighter.
Some people tie the knot below where they hold the tail end.
This stops the loose rope flapping, but avoids any problems with the rope catching. It is easy to undo because there is no pressure on it. Slip the tail end up the sleeve of your jumper before ringing. This presupposes it is cool enough to be wearing a jumper. It is best suited to ringing small bells, since moving up and down the rope as you need to on a heavier bell works it out of your sleeve.