Unused twisted pair termination with RS-485 on 8P8C
You must provide a common potential reference, so you need at least one pair for simplex data and one wire for common/GND. Just because it's differential transmission doesn't mean you only need two wires.

The power supplies to the nodes should be floating, and the 0V of the floating supply should be connected to the 0V on the cable. That way the node gets referenced to the master's 0V potential. For electromagnetic compatibility reasons, you should put a 1Meg resistor and a 1kV 10nF capacitor between 0V and chassis on each node, unless your nodes are truly isolated and have no exposed metal.

There's nothing wasteful about using Cat5e cable. It is the most ubiquitous type of high-performance balanced data cable you can find. You can buy it in grocery stores, after all.

Sidebar: If your RS-485 receiver chip is isolated or hooked up to a power/data isolator directly, you could do tricks with just using one pair and locally deriving the reference voltage, but that's making things way too complicated when you have the wires available.