In fairness to Ian I don't think he mean't to say air but he is right, many users find switching the printers off and on every day does help to keep the nozzles clear.
I can understand why this works, but it will consume ink every time (it's a bit like doing a re-prime daily). If you had a set of cartridges in your printer and did this every day then the cartridges would run out of ink without printing anything, so some ink must be pulled through. I was curious to know where the air came from as the cartridges should be filled with ink and there wouldn't normally be any air in the cartridges.
