My AC has not been cooling or removing humidity efficiently. Yesterday a technician cut a hold into the cooling coils (the original installation did not have one). He found a solid mat of cat hair and debris that had been accumulating for 18 years. He cleaned it out and I am amazed at the air flow I now get out of the vents. The AC unit also cycles now rather than running full time. I asked him whether I should use filters on the cold air return, even though there is a furnace filter on the basement unit. He thought it might be a good idea.

I used to put filters on the vents to keep crawlspace dust (my ductwork used to have small gaps) from entering the house. The service tech at that time told me to remove the filters, as it greatly reduced the air flow.

If I put filters over the cold air return, I realize it will reduce the air flow, but would it affect the efficiency of the system? Or is it just better to clean the coils more frequently than 18 years:)

The more filtering you do the more restriction you make. Restriction is bad for the system. I would one high quality filter at its present location and I would also have the unit serviced every year. Change the filter every month if necessary.