Cheating is giving you an unfair advantage over other players whilst actually in the game. Steam ratings are like high-school popularity. It doesn't really matter unless you have to be the center of attention and/or the hottest thing since Angelina Jolie's lips.
Since nothing is really awarded via Steam Ratings, and how it's so easy to get a 10 steam rating even if you just play a range of games for a couple hours at a time, it doesn't really matter much.
Now if you had to do extraordinary feats to become player of the week and you found a way to bypass that by idling, then it'd be cheating, especially if you got rewarded with in-game content. A LOT of people idled to get the TF2 hats way back when, but when Valve found out they reset the hats of all the idlers and gave the ones who didn't idle for the hats an exclusive hat.
In short, Valve doesn't like idling when you get in-game items for it, but it can be handled if nothing is gained from idling other than steam rank. At least so far.