Both of my custom bikes have slightly sloping top tubes. I think my newest one has a 6 degree slope.
- The disadvantage of a sloping top tube is aesthetics (for those who like level top tubes)
- The advantage is allowing less saddle-to-handlebar drop without making other wacky changes in the geometry.
I think "lower step over height" is a poor reason to create a bike w/a sloping top tube. Unless you're going with a *dramatic* angle in the top tube, I don't see how it would make that much difference when you're getting off the bike.
One of the other changes that a builder can do is to make the headtube extension above the top tube slightly longer than "normal." That allows you to get the handlebars higher in relation to the saddle without having a huge stack of spacers.
In the end, though, you should get the bike that makes you excited