I don't do the second pass very often, Eric. I will, however, often use masks as you mentioned to make sure that I am only applying noise reduction to the areas that really need it, usually the bg areas or large areas of solid color.
I'm still learning so sorry if this sounds daft, but I have done a similar thing when I wanted to apply Neat -Image to only specific areas. I simply brush over the detail areasand areas that didnt need cleaning with history brush directly after applying neat image.
Basicly I'm not sure how much more effective usingmasks is? And secondly I dont know how to use masks yet anyway:lol:. Lots to learn in photoshop for me.