I think it all depends on what material you use for the "omni" part, and what design you use.
For instance wheels like
this and ones like
this virutally don't slip at all, even though they have a different design. The first link is one 1018's robot (my team), and we didn't climb the step, so all we needed was grip on carpet. The second did, and they need the grip on the platforms.
You should find a design, then get materials that will best suit you traction needs.