I've used cat5 for 15 years and run 5V LED's. I double up etc the un-used lines... just made up your mind how you will apply UNFORMALLY the different colors of the wire. CAT 5 works well!
I think a well done post on this subject was made by.... maybe Kieth. I really don't remember who did it, but it would be helpful to Newbies and Oldies. (LOL)
Also please keep in mind that some of the "connectors" supplied by Chinese suppliers have corrosion within the cables and connectors (not copper oxide: green corrosion, but black/gray colored stranded wires). This corrosion will result in a large Voltage drop for your models.
BTW: I'm a Ph.D in Nuclear engineering. Doing the best I can with ohm's law.