There's no point at all to terraforming Mars if we don't include some life somewhere. From beginning to end, the three stages with which we could introduce life to Mars.
Antarctic Lichens
Antartica. The coldest place on Earth. And it's pretty dry, too. In fact - and this may seem slightly contrary to common sense - Antartica is officially a desert. And one of the coldest and driest places in Antartica is the Linnaeus Terrace. And yet life is still present there, albeit in the form of algae and lichens living just below the surface of sandstone rocks in the area (Which just goes to show... something...)
Hmm... very, very dry, unquestionably cold... sounds a little like somewhere not a million miles from the topic of discussion here... i.e., Mars. These tiny examples of just how tenacious life can be, adapted to survive to cold and dry, net consumers of carbon dioxide... is it possible that they could survive on Mars?
Of course, by no means are these little lichens the edge of our possibilities for Mars. But in the early days, before and during the time when we start pumping up the temperature and pressure, it would just be interesting to see if life could survive these harsh conditions... They would also provide the basis for life on the new planet, and who knows? We may even see little bits of adaptation and evolution in action. And of course, there'd be a little more oxygen around.
Even once Mars is up to a decent temperature, it will still be a bit chilly. We're therefore better off with plants from tundra and boreal areas - such as pine forests and alpine meadows. Slowly the surface of Mars would turn from red to green...
Once the plants are settled in, we come to the problem of animals. And they are a problem. As you should have read under Atmosphere, if we add loads of nitrogen and oxygen to the atmosphere to make it like ours (correction: like it should be...), we then return to the chilly Mars we know and hate.
So, for now - until we find some sort of answer, at least - we need animals which can tolerate high concentrations of carbon dioxide and low levels of oxygen. Don't hold you're breath, though - we're talking things like worms and insects. Well... it's a start... and the worms'll help with the soil...