Actually, anything above 2.5w/g should have CO2. If you don't, you are going to run into some algae issues, and the only way to eliminate it is CO2. BBA is a big one for that, loves low and fluctuating CO2 levels, and no CO2 as well. If the tank is ssmaller, then you can use Flourish Excel instead, as it's a carbon suppliment.
As for being careful with CO2, not really necessary IMO. I run my CO2 at 50ppm-78ppm regularly. The only time you need to be concerned is if it's running up around 100ppm and higher. Currently I'm running my 29G right around 50ppm.
Lastly, in very moderately to heavily planted tanks, you SHOULD run an air bubbler at night. Or anything that will give more surface aggitation. Plants produce O2 during the day, but take it in at night. I've lost fish due to suffocating, I woke up and half the fish dead and the other half gasping at the surface. Within 10 minutes after adding an airstone, all was normal and had to fish out the dead fish. And it included 5 Roseline sharks.
So, remember, plants PRODUCE O2 during the day, and USE O2 at night. The more plants you have, the more O2 that get's stripped from the water.