Thanks Unichef, I mostly go for a softening and try to avoid caramelising the vegetables at all, which is not to argue against your technique, as it is to state mine.
The reason I do that is partly based in this post by Luc Hhttp://www.cheftalk.com/forums/239613-post14.html
It just happened that I had the time to play the other day and was preparing a mirepoix on quite a low heat, when I wondered how completely the liquid could be replaced by the medium used for sautéing/sweating - which in my case was oil.
I took it way past the point at which I normally would, massively reducing the volume of the ingredients, but still avoiding any obvious caramelising - although there was some evidence of catching on the bottom of the pan due to inattention and/or lateness in adding some more oil when needed.
Certainly the stock was darker than usual, possibly sweeter - although I'm not very good at identifying differences from memory and didn't have a direct comparison stock to hand.
The whole thing raised the question of to what degree if at all an experienced hand might finesse the ingredients at this stage to get the most from them? Possibly even making a deliberate choice to deliberately stop at a point in order to avoid releasing an excess of one or more aspects (like sweetness) of the mirepoix.
I lack the palette to usefully make these judgements as I go, but I am interested to know if these adjustments are part of the toolkit of the experienced chef - or is this stage of the mirepoix more function than art, and not something that should be given too much weight - with any adjustments left to the later stages of the intended dish.