Based on my experience with 2 nylon boats "Your stressing to much". The nylon is going to go on tight. When you get it on the water it is going to loosen up and may loosen up pretty badly. You will feel sick and the next day you will notice it is tight again. Over the summer you will notice it less and less because it is loosening up less and less. After a year mine still will get a little loose as in not drum tight, but it doesn't wrinkle anymore. If I didn't point it out to you, you probably wouldn't notice it loosening up either.

I just did a boat in polyester and it doesn't loose up when wet. It just loosens up when it gets cold and I shrunk this stuff big time. I have about decided to just stick with nylon since it is more puncture resistant and just let it wrinkle at first and know that it going to quit it after the first season.