Henry is 'right' to a degree but the problem you are talking about isnt a REAL problem. ( I've dealt with roofing, roofers, builders, ect) The ONLY time the wrinkling would be a problem is if you are using a VERY economical shingle or over-layment. There is a product that wont wrinkle if your pocket book can stand it. One product is called Nail Fast mfg by TAMKO. GAF has one also but I cant think of the name off the top of my head. Are you thinking of 'drying-in' and waiting for over-layment? If money is the reason for having to wait save a little langer and cover it with shingles w/o felt, that'll save you a few bucks. Nail Fast is a very good product but NO type of underlayment is recommended to be exposed for a extended length of time. Hope this helps.