The correct way of cleaning lens by the pros should be:
1. Check focusing with Collimators first before disassembly, note current setting/positions.
2. Disassemble lens for cleaning.
3. After cleaning, assemble lens and test on collimator that the settings are still the same as per original. Otherwise, adjustments are to be made back to original setting.
However only P&G (Adelphi), apart from Canon has collimators. So if you sent to some other non-professionals, they may humtam and screw up your lens focusing, or they will simply act as middleman and sent in to P&G anyway.
Some weeks ago, I enquire the cost of cleaning at P&G and was quoted the same price as what Canon service centre quoted me. Since I wanted to be safer and the cost are similar, I sent in to Canon instead. I not too sure about these shops but if there is miscalibration problems after cleaning done at Canon, you can send in your camera and the lens again for them to make sure they work well together.
Last but not least, a little dust is normal. Any zoom lenses without dust is impossible. In fact, new lens also have dust. And according to some feedbacks, lens may come back with more dust elsewhere than it orginally has after cleaning! Imaging spending about $80 on cleaning, the next day you go shooting in the outdoor, dust back again! More often than not, they wont affect your image unless it is really big big.