Having only done it twice, I am far from being an authority on the subject. If I had to do it again, which I may, I wounld only stretch it tight enough to lay down without wrinkles. Once it is applied and sets in the sun a week or so, it will shrink, and if initally applied too tight, it will pull up from the drip rails.
I used a can of 3M adhesive and rolled it on with a small paint roller. Take the time to mark the roof centerline and be very careful to assure the seems are straight front to rear. Shrinking and crooked seems are the two biggest flaws I have seem with do it at home jobs. Both of mine turned out very well.