I use carbon in my external canisters pretty much all the time, changing it once a month (if it was inert after one or two weeks, I'm sure the manufacturer would be the first to tell me while looking at my wallet lol).
I've tried running it without on a planted tank, and the water became less clear. Floss will remove fine particles, whilst carbon removes impurities (chemical or elements). I just find it helps my water quality.
You can only make your decision based on your needs, and on what people have experienced. I know a lot of people don't use carbon all the time, but as I say I never had a problem and rumours that it 'releases' back into the water the impurities it has absorbed is thus far unproven.
Why don't you try it without, and then try it with (a month a piece or something) and let us know what you think?