I clean them.
On the first clean ,go around the house JUST do the vents thouroughly, then do the glass, then on subsequent cleans they will no longer be a problem.
As long as you give them a once over with the frames every month they will stay clean..
Try to get the customer to have a look, one clean one and one dirty one , explain what you have done and why.
They should give you enough time to get it right.
Practice makes perfect.
I clean the vents everytime without fail.
Dave