Thanks to all who replied. I truly appreciate the time it takes to answer my broad and general query. Let me answer/clarify a few things...
Q1: I employ a housekeeper so I don't have dust in room or house lol. My camera has in-built sensor clean that activates when the camera is turned on and off. Thus far it works very well. On occasion I have had to PP a spot now and then.
Q2: I have noticed some irregularities, but that does not necessarily mean there are. After all, it's subjective in my opinion. I am under the impression that a "check up" is part of a preventive maintenance program in that I'm preventing some kind of "total" failure before it happens by having it checked. After all, any type of equipment/machine with moving parts would be subjected to malfunction at some point (ie wear and tear).
Q3: I would agree that on my camera (Canon 400D) noise shouldn't be obvious at ISO 200. At ISO 400 it has always been there and never (even now) at ISO 100. The in-built noise reduction (according to the manual) kicks in when for exposures requiring 1 second or longer. I would prefer not to have to pp noise on images at ISO 200. I'd rather buy a better camera. But since this is only a recent issue I thought I'd ask the good people of ClubSnap for their thoughts on what may be the issue.
Additional info:
The noise is not that apparent if not looking at the image 100%. I am not the only one "pixel peeping" my images. I submit my images for sale and only recently have had images rejected due to noise. So even if I didn't know how to look for noise before, I'm sure these other people knew. I've also noticed, going through some images trying to find a few comparison shots, that the noise only appears on images with a lot of sky and that's where the noise is. I tried shooting some product (still life) shots at same settings but didn't get any image rejecting noise levels.
I've posted a pp'd image here:
http://www.flickr.com/photos/webitect/3418706479/
and a un-pp'd image here:
http://www.flickr.com/photos/webitect/3421483076/
Not sure if you can see the noise though.
Once again, thank you all for the input.
You mix 3 topics into one question.
Q1: Dust
It's the same as with your room: when does it need cleaning? Apart from elderly complains or advices you can tell from the layers of dust settled down on everything. The same goes for sensors. Search the forum for 'sensor dust' and you'll get plenty of answers about how to check and how to clean.
Q2: "Tune up"
The camera is neither a Windows system nor a car, it doesn't need any tuning. AFAIK, there is no "preventive maintenance plan" as for cars or aircrafts. You use the camera till you notice irregularities. Then it's time to see Canon Service Center. But you can bring in earlier for a general check-up also.
Q3: Noise
Noise is an error signal appearing in the image, caused by amplification of low light signals in the sensor. It's a characteristics of the sensor that cannot be changed. Cameras have built-in noise reduction or you do it in post-processing. Post a picture here with full exif data. Maybe you haven't noticed before because you didn't know to look for it? But at ISO 200 there shouldn't be any noise.