I believe if you have a plane for the return current the highest density is actualy at the bottom of the trace, for the uninitiated we are discussing skin effect.
This argument is almost academic, as for most high speed digital designs (lets leave GHzs RF out of the equation for now) as you are best to run your traces as stripline on innerlayers so they are not gonna get plated. I have never employed any plating on traces or had to worry about solder resist for any HS digital design yet, and as said earlier I try to maximise stripline routing for high speed signals as the wave propagation is nearer to true TEM.
Very high GHz RF design is a different ball game...