No, it won't make any difference if a Canon technician cleans it. Mine came back with two technician buggers on it, really large, and a pubic hair--probably beard. Sloppy.
I agree about the 5D leaking dust or mirror box soot all over the place. I can clean my 5D and have it perfect at f22, only to get specs without even changing the lens. Open it up, use a rocket blower, and they are gone. This tells me something in teh box is contaminating the sensor. Next time I clean it, I'm going to clean the entire mirror box with my new set of cleaning swabs and Eclipse--they now come with those items as standard for a package and recommend cleaning the mirror box and inside of the camera well.
And be prepared to clean your sensor for the rest of your ever loving life, and be prepared to specs sometimes even after you clean it. It's just a fact of life with digital. It's nothing the patch tool doesn't handle perfectly and quickly in Photoshop too.
I use to be really anal about this subject, but now I jsut clean it the best I can and shoot. If there is a spec noticible in the image, I patch it in PS. I don't have the time to worry about this crap anymore.