The only time I got a commendation as DPS was when the tank in an AV apparently lagged and got dropped by Coin Counter's 100t swing. I ended up tanking it for more than half the fight(thankfully most of his abilities are avoidable). Aside from that, I only ever get commendations as my warrior. And even then there are plenty of groups that either give them all to the healer or just don't give them out at all.

I always give mine to the healer as a tank. If I'm dps, I alternate between tank and healer(unless one or the other does an exceedingly poor job).