Or don't do it at all. Here is a perfect example of why the job you do as a kitchen exhaust cleaner is so important.
Smoke Explosion Kills Two Boston Firefighters
A sudden fire development that occurred in a Chinese restaurant fire in the West Roxbury section of the city killed two Boston firefighters on 29th August 2007. The fire, that appeared to originate within the ceiling space, was fuelled by a possible build-up of cooking grease in the ceiling area or grease duct.
Full story here: Smoke Explosion Kills Two Boston Firefighters
Property damage is one thing, but the lives of firefighters is another. If you don't know how to get your systems cleaned to bare metal every time, get some training. There are several free resources if you can not afford the training. Ask questions, ask guys for help, there are plenty of guys around that have been through every situation imaginable and can tell you how to get your exhaust systems clean every time.
These fires very rarely start in the ducts, malfunctioning kitchen equipment or operator error causes most of these kitchen fires. All you can do is make sure that every bit of combustible material is gone from the systems you service.
Do not leave it up to chance. Be proactive. The next restaurant that goes up may be one that you decided not to fully clean for one reason or another. Imagine for one second that the restaurant in this story was one of your accounts. Imagine looking in to the faces of their loved ones and asking for forgiveness. The choice is yours.