If you don't want to risk detergent residue, you can still backflush with water, and I would do so on a daily basis. I was just helping a friend with a Classic just purchased from Craigslist, and a few rounds of plain-water flushing got the (initially completely blocked) solenoid back in functional shape.

BTW, Amazon has "Espresso Supply 902070 58mm Backflush Insert, Set of 3" for $11.50. (Eligible for free shipping or Prime, too.) Great deal there, at least if you want to keep more than one around.

Just spoke with a sales rep at WholeLatteLove while trying to buy a blind filter to backflush my new Gagia Classic. He strongly recommended against it, saying that the three way solenoid valve was not designed to handle the amount of pressure on it from so much water being pumped while backflushing. He said he could sell me the blind filter and the powder and then sell me a new solenoid valve for $50 in a couple of months when mine would break, or he could just save me the $30 up front. Also, he made the point that water is being forced out of the puck and not into it, so there wouldn't be any coffee oils that get inside the machine. He's on the front line and seemed honest to me, so I just went with buying the descaler instead.

This has been discussed online many times before,always with a recommendation to ignore the advice. The general explanation for Gaggia's position on this is concern over warranty issues. In the Gaggia group on Yahoo, as well as here, backflushing is recommended and seems to occur without incident.

Since this thread is back from the dead, I will say that in my experience of intimately working on these machines for several years now, backflushing with detergent is not necessary. I can't give advice based on other folks experience with this machine or any other for that reason. I can only give my personal experience. And if anyone reading this does decide to backflush w/ detergent, whichever brand that is, then be aware of the risk. Happy brewing!