Evaporator Coil Cleaning - When Should It Happen?

Yes, and it does not involve very much fire. More seriously, there's a right way to clean your evaporator coil and it's something that should happen on an annual basis. Inside or outside the home, a dirty coil can reduce heat transfer efficiency, meaning your system has to work harder and burn more energy to provide the same air comfort it used to. Regular evaporator coil cleaning, the right way, is key to maintaining system life and effectively managing your energy consumption.
Evaporator Coil Cleaning With Fire!
Or not.
Actually, the easiest thing you can do to keep your evaporator coil clean is to keep your air filters clean. In a properly insulated home, all the air passing over the evaporator coil should've come past your air filter first. Air filters do a worse job the dirtier they get, so if you don't change or clean your filter like you should, the coil could get dirty. On the upside, if you're on top of the filter situation in a well insulated home, then your evaporator coil probably isn't that dirty to begin with.
OK, it's dirty, what do I do?
Depending on how easy the evaporator coil is to access, cleaning the coil may not be that big a deal. Different evaporator brands are, well, different, and some coils are easy to get to and some aren't. First, your tech will open up the panels to determine how easily they can access the coil. And, yes, you do need to get a tech to do this. Evaporator coil cleaning isn't exactly rocket science, but you probably don't want to violate your warranty.
How would you do that? Most HVAC warranties require that only licensed technicians install or perform maintenance on the equipment. It's not that HVAC manufacturers don't trust the average person, but, well, they don't trust the average person.

So, what is the tech going to do?
The tech is going to come to your house, disconnect the power to the unit, and open up the panels. If the coil is fairly clean, they might wipe it down with a damp cloth and some soap. What they definitely won't do is apply any solvents that could cause damage or corrosion. If your coil is really filthy , they will use a spray-on cleaning solution to remove the dirt and debris.
Above all, they're going to take care of your and your unit. You'll get a clean coil and better efficiency for only a couple bucks. Which is not a bad deal at all.