Easing the diamonds reduces the preband, makes the sail fuller (at the same downahul and main tension). Depending on your sail cut, you may or may not want to have a full sail. Probably you may not. More important in light wind is the twist of your sail, hence watch your tell tales.
For me the most important thing and I have always to recall it is that once the boat is going, let it go and concentrate on helm and sheet.

Cheers,

Klaus