I think its inevitable sound editors build up a mix as they work, because its essential to knowing that their material is working... And when you present your work to the director for input you want it to play as well as possible ie as close to the final context in the film...

But in a final mix a lot of the work involved is balancing overall groups of elements (eg dialogue vs score vs ambience vs effects) for best dramatic/emotional effect so in that sense I think as a sound editor I tend to more build up premixes than a version of the final mix.

Towards the end of the schedule I will have temp versions of the actual score to work against & a good dialogue/ADR guidetrack, and I bring all my elements into one session so I can build up context (ie all ambiences, effects, design elements etc) This also helps the director to think about how each scene will play in the final mix...

I dont attempt to do a pretend final mix while editing though - I am focused on getting the individual elements as good as possible, and working out how many stems we need to premix to etc.. but I do work with VCA faders in ProTools, so during a run through with the director if there is an element they want to hear relatively quieter (or louder) I can quickly show them...

When we start premixing I usually ask the mixer to play the scene we are about to premix with 'flat faders' so they can hear the balance I built up while editing & then follow their instincts on how to improve on it. This objectivity is why I personally prefer to edit & not mix - a good mixer brings a lot of creativity & objectivity to how your work will play in the final mix...