RJ;
I essentially follow your method in that I cruise on the surface as much as possible and dive if there's a perceived threat (usually an air radar contact). Even after a radar contact I'll generally push my luck and wait for a visual sighting before crash diving. Having said that, with the TMO mod, the crash dive takes me deep as the aircraft can still see you at periscope depth.
As RR states, cruising on the surface, even in the daytime, allows me to cover more of my patrol area and saves my batteries for when I need them. As soon as I dive I'm very conscious of what's happening to my batteries and when I need to recharge.
