Depends on the boat. On boats without boom like a Dart 18 it is very important. On boats with boom it is less important.
If I minimise the weatherhelm on upwind courses, my boat is very easy to fly on one hull, but on downwind legs with and without spi the leehelm is a bit strong. I balance between these two effects. I didn't measure the speed, but I don't feel a difference.

Cheers,

Klaus