Weather watch isn't as good as Grace Ice & Water, but I guess you have to stick with GAF in order to keep the warranty. In that case, I would suggest using Stormguard. Also I believe it should be 36" over heated areas, areas that aren't heated shouldn't count which is why you should just ask for 6 feet. Also instead of shinglemate, just have them use deck armor, probably about the same price.
And while the nails might be an extra $50, it's a bit more labor putting in those two extra nails. Not sure how bad storms are in your area, but if you get some good ones, it might be worth it. Weather seems to be getting more and more extreme as the years go by. And if the guy is certified, make sure you read the GAF warranty and make sure the paperwork from GAF shows up after the job is done.