I guess it depends upon how seriously you want to waterproof your hats. Around here it rains so seldom - I'm most likely found standing "out in it" when it rains.
However, I will use Scotch Guard spray on my ball caps. If you are dead nuts serious - go to the Aerostich web site and look at their waterproofing products - Nikwax.
I use the Nixwax laundry products and waterproofing sealant products on my textile motorcycle jacket. One treatment will last me an entire year of riding.