Ratchets...

The ratchet system is a great thing to understand and use.

Light air... ratchet off.
Allows the sail to sheet out easily and quickly. Usually there is not enough pressure on the main to pull the line through the blocks with the ratchet on in light air. Allows precise sail control with instant response.

Medium air and light crew or single handed... on or off?
I would keep the ratchet off as you need the sails to respond quickly to healing motion. A ratchet on in this condition could delay sheeting out and cause a capsize.

Heavy air and plenty of crew weight... ratchet on.
The ratchet spins easily when sheeting in, but locks when the sheet is released or held. The line now has to drag across the sheave. Harken blocks have little flats built into the sheave to help pinch the line and hold it when the ratchet is on. Old Seaway blocks have ridges in the sheaves.

This greatly reduces the pull against your sheeting arm.