I don't wear a brimmed cap as much anymore, as I do prefer no brim, for reasons mentioned above about having to crane neck up that bit more to clear your view from being blocked by the brim.
The Giro UV Ultralight SPF30 skullcap works well.
Otherwise, for just a bald spot in the back.. use the aerosol sports sunscreen.. easier and less messy than trying to rub lotion up there. If you need more coverage all over the top of your head, then yeah... watchout for stinging eyes if it runs.