When I picked up my 370, I had been told it was valet'd. Interior was crap, and even found all the garage window stickers under the passenger seat. General rule of thumb for claying, is after you've washed it, to run the back of your hand on the paintwork, and if it still 'feels' grubby, claying is probably a good idea.
There are vids on youtube that help if your unsure. Makes a hell of a difference believe me!
P.S. Am I the only one who finds it theraputic?