I don't put anything under the track on my hardwood floor and find no scratches.
However, depending on the slipperiness of your hardwood floor you may find the track tends to move around a bit, especially at the corners. If you use slip-on type track connectors you might find the track starts to separate a bit, especially at the curves. A simply bit of hand tightening, i.e. pushing rhe track together, will be sufficient to mate the track pieces once again.