Lost,
Years ago, I had my mainsail cover made with extra 6" of extra material vertically past the boom. Actually, I think it looks neat also. We get the occasional tropical storm/hurricane & I wrap line around my cover until the BLO passes. I've seen boats with tight sail covers & if leaving covered for a time, that ol green/black mildew stain would rear it's ugly head.
The secret is to use a looser fit fabric that allows air to move under the cover, thus drying moisture in the sail. I've had a looser fitting cover the past twenty years, & have never had mildew stains. Here in Florida the fall & spring leaves alot of overnight moisture & I don't want to keep my sail wet under cover.
CR