It depends on how tight the down haul was when going upwind, and what the apparent wind will be once the spinnaker is up. If it was very tight going upwind and you’ll be going very deep downwind, ease it to gets a deeper draft into the main, just remember to tighten it again at the C mark before you turn upwind again.

The goal is to maintain an optimum mainsail shape, usually flatter going upwind and fuller going downwind.