In most high performance PCB designs a backplane ( or power plane/s ) are usually recommended. However there are reasons not to use these under components sometimes if better performance is to be obtained ( or oscillating amplifiers are to be avoided). So do not use backplanes ( ground or power) under components that may be sensitive to parasitic capacitance. For example a high speed opamp will oscillate if a backplane is used under it ( or its inputs). The reason is that the backplane leads to the generation of a parasitic capacitance under a pin which in  turn causes a loss of phase margin. Additionally parasitc capacitors cause loss of bandwidth or high frequency or high speed performance. Therefore it is best to evaluate where and ( where not) to place a backplane. Some early consideration to this can avoid headaches after the board is fabricated.