Answering as a teacher and a mom.
Teacher...as I teach middle school, no, I don't change what I say. I like the kid to be there, it is about taking responsibility for their own learning...many times though,
If it is a "team conference" involving all major subject teachers, we talk to parents first and then bring the student in for the second half.
As a mom, I wouldn't want my kids there for the whole conference. I want to hear it straight with no positive spin. I have a kid with severe adhd, so there is a lot to say! I sometimes bring them in for the second half of the conference...usually at that point it is more positive. Kind of explaining to the student what they are doing well and a plan to help them improve areas they are weak.
Conferences with kids should revolve around plans for improvement and praise for things they already do, not the time to tell them all the things they are doing "wrong." It is very intimidating to have the important adults in your life analyzing everything you do and don't do. I've seen 8th graders break down in tears before a word has been spoken. So a lot of this depends on your child and his/her personality as well.