Just registered to suggest this, because it would improve working with progrids so much for me, and since it's going in the same direction I'll put it here. Maybe I'll get it soonish..? Should not be hard to implement, maybe as an option?
I really like using the + and - keys, but they have a mayor flaw for me: they scale horrible. I like working on a 1 meter scale most of the time, so scaling down leads to 0.5, 0.25, 0.125, ... this gets ugly really fast imo. Also it's very different from the values when scaling up, which never hit 10, 20, or anything "clean".
My suggestion is this: since this is basically a logarithmic scale, use the common minor ticks for logarithmic diagrams. I always used 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, and so on. They are nearly equidistant on the scale and repeat nicely. Should be pretty easy to implement and I would use it all the time without ever going back.