This message was posted by Gus Pratt on the Onelist TUBAEUPH discussion list several years ago. It is still a good discussion.
When tuning the valves the most critical thing is that you think about whats going on with the horn from a techincal aspect. If you tune the first valve first, then when you adjust your main tuning slide the note you tuned with the first valve is now out. Here's the order that I would use, some of which will need a tuner.
1. Tune to concert