It is if your choreography calls for a straight line. My partner and I have problems with under-rotation too. I recommend looking at this thread where a lot of advice has been given on how to fix that: http://www.dance-forums.com/threads/pivots.38290/
Even if your routine calls for pivots down the line of dance, a SLIGHT curve to the left is fine and probably a good thing. If they curve strongly left, then there are probably some issues with how you and your partner are dancing them. From the leader's perspective the "pivoting" mainly occurs on the backward step, while the forward step is much more linear. So the leader needs to take his whole body back to his left foot (often leaders just take their top and leave their hips in their partner's path) while rotating with their body over the ball of the left foot. It is then important to allow enough time for the follower to complete her forward step and to rotate to the desired alignment before the leader takes his forward step. If the leader is off balance or impatient, his forward step may be too soon and will result in under-turned pivots. This is just one possibility.
