In general my favourite setup is 9" on the rear and 8" on the front. keeping the 8" on the front makes the board more nimble than when you slam 9" all around. This setup is a great compromise for all type of riding.
8" or even 7" on all wheels definitely is better for pure on road use, but they greatly limits what you can actually do off the road.
Overall though i have to say that the 10" tyres all around really ruined both 9 an 8" for me. Those fookers just go over stuff like it aint there. They sure make the board heavy and clumpsy but shit is it hilarious to surf over whatever the heck you want! I tried to put the smaller tyres back on the board but when out riding i found myself constantly wanting to hit those gnarly forest paths that only them BIG boys can handle. Always in the back of my head "why did i change back to theese peasant rubber infants.
I really need to get that 1:6,25 ratio instead of my current 1:5 while using the 10" though. it currently tops out at 75kph which obviously takes a huge crap on my low RPM torque.