
If you turn over two bikes, one with Hope cartridge bearings, and one with XT cup and cone bearings and spin the wheels, the XT one will spin for longer.
But, as the drag of offroad tyres is generally pretty huge in comparision I doubt you'd notice it. Even on my commuter bike with thin slick tyres the biggest drag is wind resistance, especially once you start going over 18 mph, as I find there's a noticably huge difference in effort required to maintain a particular speed which depends on wind direction.
For a mountain bike I don't think the higher drag is really a problem.