Thickening wire to prevent voltage drop
The 'power momentary switch of a PC' should take very little current at all, at least with respect to cat5 cores. It is a logic input to the mobo, so a few mA at most, probably &lt; 1mA.

If OTOH there is significant current running through it, then I would not use cat 5, even multiple cores of it which would 'thicken the wire'.