[PlantedRich]

If the cycle has been done and the filters are ready for the current load, I would not!!
Carbon has it's uses but it is also a big pain to deal with on a routine basis. It doesn't last as long as the rest of the filter media takes to get dirty so it doubles/triples the number of times you have to open and clean the filters. It fills up and stops working. It is a filthy mess to clean out and dump. Then the new is full of black gunk that requires rinsing. The final kicker for me is the way it works when you need to treat a fish.
Say you have a sick fish, the last thing it needs is stress but before you can treat it, you have to tear down the filter and get the carbon out. That's like being in bed after surgery and they come in to change the sheets! Otherwise the carbon will remove the medicine you are trying to use. A couple good filters like you have will be fine without the mess, work, and expense of carbon.
I lay the carbon pads, etc. back on the shelf for any time when I DO want to remove colors, smells or meds. Rarely happens.

 

[PlantedRich]

I often go with the question of what value each item brings and balance that against what price I pay on the other end. With carbon, I just rarely find enough value on points that I can't cover in other ways. With a couple good filters, you have a vast array of ways to change up the media slightly to cover the three things filters do for us.
Then if I found I had dingy water or water that smelled, I would look at what I was doing wrong in other ways that cause my problem.
On the fert question, I don't feel it has a large enough effect on ferts to really matter, either way but on all the other points, I'm not a carbon user.