I have creased sails. Is it safe to iron them, or is there a better method?
A very, very low-heat iron should be fine, although you will have to bear in mind that some of the sets of sails have stickers on. I would advise either going around the stickers (the heat could cause them to melt or peel off) or ironing the back of the sail.
I guess the material for the sail is the same as the part from 6091 - King Leo's Castle - which has been surprisingly resiliant to wear and tear over the years.
Another option would to be place the sail under a stack of heavy books for a few days.