"Step over height" means absolutely zip to me in terms of mounting - I always swing my leg over the back of the bike (and the saddle) rather than over the top tube. That saddle height will be the same distance from the ground, regardless of whether the top tube slopes or not.

"Stand over height" is a different animal to me, and IMO implies the distance between your crotch and the bike when you come to a stop and get off the saddle, but stay astride the bike.

The only reason I'm planning to get a sloping top tube bike is that all of the "relaxed" road bikes I've seen on the market (those with a shorter reach and longer head tube) seem to be designed that way. If I was ordering a custom bike I'd probably rely on the advice of the designer.