I really prefer the hands be on my lower-back/sides under 'normal' riding conditions, since I think you both feel more 'connected' and it's easier to lean together. Like Kathy said, it's easy to shift one or both hands to the tank when necessary for harder braking/more aggressive riding.
Just don't let her get
too comfortable, or she'll be nodding off to sleep - I hate it when dem bitches do dat!