Pad is a good idea. Two reasons. One, more comfort. Two, the runner will degrade(flatten out) without pad. It can be a thinner pad. Stair carpet takes more stress and strain than flooring carpet. Feet sliding, body weight on each foot for a longer time period, and twisting.

we're getting ready to replace our runner, and just today i looked at our current one and slid my finger under and there is a pad.

i'd believe the advice to get one.