Originally Posted by msmith10
The only caution with tying a dock line to the toerail is chafing. The edges of the holes are sometimes a little sharp.
I don't think that is the only caution. I'd use my cleats for most dock lines, all if possible. The constant loading and unloading of pressure a dock line can develop in bad weather could help pull your toe rail out or damage the hull/deck joint, if the rope didn't chafe through on the rail first. For a quick tie up then sure, use the toe rail as well if you like.
As Jeff_H and others have suggested these toe rails were intended to be used for attaching working loads in the running rigging of the boat (the sails).