Strictly speaking the entire OS should be a consistent version/revision, so OS3.1 should be KS3.1 / WB3.1...
Having said that, WB3.1 ran ok on my KS3.0 A1200. There may be some small incompatibilites that lead to unusual instability but I can't say I ever noticed any.
If you intend to go 3.5 / 3.9 then KS3.1 is a must. I'm not sure if anybody is using 3.5/3.9 on KS3.0 - perhaps it's possible.
