For any catamaran w/o a spinnaker, you do.

Witness a Hobie 16 in optimal downwind trim:


The main needs to have two tell tales up high (if you look closely at the photo, there's one right above the sail insignia and another in the panel above that) to set the twist.

Twist in the main (in varying degrees, depending on the wind strength and sailplan) is almost always needed to account for the vertical change in apparent wind.