Data Bus and High Impedance
You only need to set a bus to high impendance if you have more than one device with drivers for that bus. It doesn't matter if this device is a processor or a memory.

In the case of multiple devices being capable of driving the bus, it is an error if more than one of them do this at the same time. That's why data bus drivers usually go to high impedance after being active.

Many processors are able to put their address bus into high impedance, too. This is necessary for example if you have DMA devices, DMA = direct memory access.