My mother bought me my tuba in November of 2009. I did consult with my teacher about it (as well as a few other tuba 'feshnulz), but that purchase had the added condition of time. I needed a tuba quickly, so we didn't have all the months in the world to scout out the best tuba for the best price, take time to play test different kinds, etc.. (I still think I got the best tuba I could have gotten, though.)
Talking to the real issue at hand here, though, I believe that a private teacher is the best way to go about
both: following instructions and listening to suggestions. There are times for both and it just depends on the issue at hand. I think it will also depend on how well you and the teacher know each other, how familiar you are, and other teacher/student relationship factors that don't have anything to do with musical or teaching skills because that will definitely influence a decision as big as the purchase of a tuba.
With regards to issues of musical performance, I usually always go with what my teacher suggests (and I like that his style is about giving suggestions and not orders). I trust him to know what's going to give me the best result and I'll take it up.
It has been with great anxiety, unease, and reluctance that I have recently decided to not act in accordance with two of my teachers' suggestions about a major decision I had to make. This was a really big "thing" and I naturally went to them first and many lengthy conversations took place where I was given a lot of information to think about and suggestions for what I should do. In the end, however, I thought the best choice for me was another one and I made that choice.
Here's the thing, though. One decision of mine that I
know I did right with was choosing these people as my tuba teachers (honestly, though, as if I needed any more evidence than the improvements in my tuber-playin'

).
Both of them were exceedingly supportive of me when I informed them that I disagreed with them and chose what I chose.