CrossRoads:
I put two on every board, with vias connecting them. Vcc traces over the plane adds some capacitance for the power signal. Hard to get any capacitance when Gnd is connected to Gnd tho.
Boards are made taking double sided copper and removing the bits around traces & pads to isolate them. Any that's left can be ground plane.
If you can fit it, it is almost always better. My last project didn't have them, but it also isn't switching anything at high frequencies or powers, and I ran out of space for traces to try and keep the board as small as possible.