Hi Wayne,
Problem you might have is if you only clean the spots they might be left with clean circles on their carpet depending on how dirty it is. If its clean-ish you might get away with feathering out around the edges so as not to leave much in the way of defining clean/dirty lines. You'll should be okay with a m/s for most spots/stains as its polyprop or a solvent for any oil/tar/gum/bluetac/toners etc. (use sparingly on cloth).
Other way to look at it is that if you leave clean circles on their carpet they migh have you back to do the lot in the very near future
. Has happened to me before.
I've just done some big offices, used powerburst for first all over clean then went over seating and desk areas and bad traffic lanes with bonnets and chemspec rotobrite, came up lovely.
All the best,
Jason.