All lithium batteries have built-in power management to prevent overcharging or undercharging. It is never good for the battery to be constantly trickle charging though.
I have the Magic Mouse and have never turned the switch off. It lasts me at least a month or two before needing to be recharged. You don't need to worry about turning the switch off when you're not using it. The only time I would turn the switch off is if you were packing them up in a bag or not going to be using them for a long period of time (several days or weeks).