If your tank knows what he is doing, if you know what you are doing, you both know how to play your classes, then dps, but never forget that a healers main responsibility is to heal. The dps should always be seen as an added bonus to the group and not a must, this also goes for tanks. Focus on your main responsibility first and worry about your added utility after the fact, using it when you see that the group allows you to.

And never ever forget that a pug is still a pug, take it easy during the first pull and see what the group is made of. There are after all tanks who can`t tank, healer who can`t heal, and yes even dps who seems to have problems figuring out which part of their weapon they should hit the mob with. Always go into a pug with low expectations and celebrate every single time you end up with a great group.