I handled and field-stripped a P5 that had exterior steel parts Roguarded, internals NP3'd. The Roguard looked great-similar to the semi-matte oxide finish found on new Glocks. I personally haven't had any problems with my personal P5's blueing, but my wife's P5C has some minor freckling on the forward portion of the slide (I bought it used; the freckling was present then). If your P5 will be exposed to incliment weather/sweat, etc, Roguarding/NP3 might be a good idea...
Best, Jon