#5 is the biggest I use in gear that I make, such as a robust zipper for a pack lid closure. I use #3 probably 90% of the time which is for pack strap pockets or bug nets. My personal approach is to only use a larger zipper where more strength is required since they are heavier and more expensive. #3 zippers can be difficult due to the small size of the pulls, but I solve that by adding these:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Of course, if weight is no issue and you like the feel of the larger zipper, there's absolutely no reason that I can think of not to use it.