Are the signals high speed or do they have fast rise times?
As I learnt recently, this can cause both signal integrity and EMC problems. The best way for power distribution is a layer for each voltage but this isn't always possible, this then means split power planes! As you are going to have a signal layer next to a split power plane layer, these signals will be referenced to this layer, and crossing a split is not good.
If you have or can get hold of H. Ott 'Electomagnetic Compatability Engineering' part 16.3 Return Path Discontinuities.
As to distributing the power, it is hard to comment without knowing the topography of your layout, current requirements, switching requirements etc. There is a lot of info coming out about "power supply integrity", its worth having a look around the web for info on it.