I am wondering if a dirty tuba affects sound in a great deal. Not regarding if it shines or not on the outside, but as oil residue etc. build up in the valve canals and stuff like that. The pipes and canals on a tuba are quite large, so how big affect does it have on tone quality?
The difference really is noticeable to an experienced player. Depending on the experience level and even genre of music played, the effects may be noticed after only a few months of not cleaning, or after two years. It all depends. If an high school player is playing mostly the low rumbly notes of a concert band piece, they may not notice much affect at all, until it gets REALLY bad. On the other hand, a college player working on a solo piece with an extended high range into the top (and above) the bass clef, would notice much sooner.
In my experience as a trombone player, after not cleaning the slide/leadpipe, I begin to notice changes in resonance, tone color and playing resistance after only a month or so. Keep in mind that that is playing at least an hour or so every day of the week, and often far more.
When I play bass trombone, euphonium or tuba, I generally don't notice the resistance changes, but that may be because I have less experience and playing time on those instruments. I DO however notice a change in resonance. A dirty tube, in my opinion, deadens the resonance, anywhere from just slightly, to quite a bit. In effect, it can make an expensive professional instrument feel/sound more like a cheap student instrument that wasn't built with the same care.
I'm assuming by the way that you worded your question that you aren't looking for instructions on HOW to clean your tuba, but more information on whether, with such large tubes, the buildup can affect the sound. In short- it can and does for most players.
Overall, the residue will affect the sound if it's large enough to change the shape of a tube. If you have a lot of dirt or "stuff" inside it can affect the tube size or change the direction of the sound waves, and so change the tone and pitch.
It is probably more important to keep the valves clean and oiled so they don't wear and leak around the edges (and also so you can move them easily). Also, you should make sure there are no air leaks at spit valves and joints in the tubing.
You can clean a tuba by pulling apart all the tuning slides and everything else that can be disassembled, and using a bottle brush and rag with soap and water. You can also run a water hose into the bell, but don't force the water through. You may have to rotate the tuba a time or two to get all the water out.
See this answer for more details on cleaning.