Proper Finger Placement on Rock Band 3 Pro Keys

It's been a long time since my childhood piano lessons, and now, with Rock Band 3's introduction of the Keyboard Instrument, it's time to learn it all over again.

The Rock Band keyboard has notches on the left-most key at the left end of each colored section (sans the outermost red and orange sections).

What is the ideal finger placement on this keyboard when playing in Pro Keys mode?

I assume you mean this in the context of both hands? This peripheral is only intended to be played one-handed, and most of the parts are charted exactly that way. Therefore, ideal finger placement is just wherever the particular chart in question dictates you place your fingers.

(Of course, some expert parts are difficult enough that you may want to use both hands, but in those cases you won't be RESTING your hands for any period of time anyway...)

There are general lessons that will show you suggested fingering for certain sequences of notes; from these and from your piano background, you may end up starting in a specific position for a specific song at a specific difficulty, but there's really no one place to start your hand (just as Pro Guitar doesn't really have one place to start your fingers).

Charts for songs on Easy and Medium generally do not require you to shift "colors", so there may be positions at those difficulties where you could set your hand and not move it much, but on Hard and Expert, even the easy songs will require you to move back and forth from time to time. The marks on the keys simply help you return to a specific color.