It's not the cabling that uses the wires, it's the networking standard being used, such as 10BaseT, 100BaseT, gigabit Ethernet, etc. The physical networking going on determines how many pairs of wires are used, the cabling just supplies the wiring along with other characteristics.
Here's some info from this page:
Gigabit and Gigabit+ LAN Notes: While 100M LANs allowed a 2-pair (4 conductor) wiring version (100base-TX) gigabit LANs require 4-pairs (all 8 conductors) which is functionally similar to 100base-T4 wiring. Even if gigabit LANs are still in the future - use 100base-T4/1000base-T (4-pair) wiring. If you still have 100base-TX (or even 10base-T) wiring in a gigabit LAN it will still function but auto-negotiation will limit speeds on those segments to 100Mbit/s.http://www.zytrax.com/tech/layer_1/cables/tech_lan.htm
Here's another page for you, that details which wires the different standards use:http://pinouts.ru/NetworkCables/ethernet_10_100_1000_pinout.shtml
So, to answer your question, 100 Mbps uses four wires, while gigabit networking uses all 8 wires. So it depends on what network you are running.