That depends... How high do your nitrates get? how often are you doing water changes?... I find carbon useful in certain applications... where I'm understocked and doing water changes less frequent ( the carbon seems to help the TDS levels).. When my bio-capacity is not keeping up with the waste out-put ( Short term fix while I either add more bio or reconfigure the filtration) .. and last but not least... After medicating a tank or haveing any form of "funky water" issues ( again until the cause is determined and addressed, The carbon is then removed after things return to "normal")
imo Carbon became "Neccisary" in all tanks when people use to change out 10% once a month... or just "top off" tanks. Now that most hobbyists do frequent water changes the need/uses for carbon has dramatically dropped. Also dechlorinators and heavy metal removal via liquids further makes carbon obsolete imo.
Carbon or the Dust from it is often linked to cases of HLLE or HITH and Fish such as Oscars/cichlids in general seem to be more prone to show the effects. Which is why many cichlid owners do not run it in their systems.. while some do.. and have no problems.. Personally I had issues running carbon with my pike cichlids, once I stopped useing it.. The fish cleared/healed up.
To sum it up in a nut-shell... know hwy your useig the carbon, if you use it.