Hi Chris,
if you refer to a simple network with short lenght cables the answer is NO. UTP-8 cables can be used for 10 and 100 Mbs.
If you have a big building then answer is YES, it's better use STP-8 cables.
The difference is UTP are unshielded cables while STP (shielded twisted pair) are shielded cables.
If you have a big building it better certificate all cables so you are sure tehy works fine and can carry 100mbs signal.
Antonio Vigliotti