I would go level as well, the brother in law had a garage done with the 1/8" per foot pitch and you really feel it just standing there, that ruined the whole garage for me.
My garage is about 10 years old and some of the rubber gaskets on the screws that hold the roof have degraded and are leaking, I really don't want to replace all the screws, so what kind of sealant should I use around the screw heads?