I have my shaker set to run on startup. I honestly couldn't tell you if it's doing anything or not. If it is, then I never see the result. If it isn't then I must live in one of the cleanest environments on earth (I don't). Once in awhile I'll get the stray spec that the shaker doesn't knock off but a quick blast with the blower takes care of it. With both the K7 and now the K5, I have never had to Touch the sensor to clean it.
When I saw the stain in the photo, I already knew it was likely (mine was one of the first built) so I simply contacted Pentax and got the ball rolling. I'm still a little concerned because my replacement is built right near the end of the period they say is at risk. I've taken twice as many shots with the new one as I did the first one however and so far so good. Just to be safe, I extended the warranty by 2 years when B&H was selling them for $20.