I think it goes into the planning process working smarter when you airbrush. If you are going to have hair for example working in a background around every strand would be hard and usually ends up in a background that effects the picture I have a couple examples I will post for you were I have made this mistake. Here I did the background first then the subject

in this picture I did the background after the portraitunless I went in and masked the whole portrait off I ended up with the light spot around it as I didn't want the black to effect the already painted portrait especially when you have long strands of hair.

I work on claybord which erases easy and if I do the back ground first I can erase the over spray. Now when I did my Roger Williamson Painting I did the background last and it was un effected as I could use shielding easily to protect my artwork. It had allot of flat edges and some round ones. It really comes down to how you plan out your artwork.