Should I turn the macbook off or put it to sleep mode?

[FieryFurnace]

... Not to mention the fact that removing the battery causes the computer to underclock.

The CPU will underclock and you might see some performance issues.

Is that also true for the "new" white MacBook?

 

[CalBoy]

Is that also true for the "new" white MacBook?

I'm fairly sure that it still applies; there's no reason to think otherwise.