I would have to say wax is too soft and not durable enough for stairs. Polyurethane is a very simple to apply and very durable built up finish. I recently built a set of oak stairs and finished them with a first coat of 1 part linseed oil 1 part polyurethane and followed that (after considerable time allowed for curing of the first coat)with two more coats of strait polyurethane.
Disclaimer: when I say polyurethane I mean the stuff in the minwax can called polyurethane which is actually almost all varnish but with a polyurethane resin added to improve durability.

Cheers,
Werner J Stiegler

Wax provides almost no protection and prevents adhesion of any other finish. And it IS slippery. I don't know why it would even be considered. Poly is tried and true long-term protection (years) on floors.

No way would I ever use wax as a finish for any floor or stair. Ever. (Unless you are a masochist!) Either use tung oil or poly. Tung is spot repairable. Poly is not but is tougher.