There are no set rules for tanking, it's a subjective test.
From the rules page:
Unfortunately, there are no set rules we can lay down to squarely say you're tanking. That's why we require a nomination AND a unanimous vote.
I think you're walking a fine line, but you're not messing with your rotations.
If the rules said that you had to do your best to win games, then I would nominate your DC. However, there is room for people to experiment with their teams, as there should be.