Depends on the network.

The only reasons to use fiber instead of copper are cable length limits (cat-5 maxes out at 100m/327 ft) and bandwidth (fiber can handle more traffic).

For most networks, you're not likely to exceed the 100mbps fast ethernet, let alone the 1000mpbs supported by newer Macs.

I might use fiber to link together multiple switches, but still use copper to individual servers/workstations.