Most things you hear or read about dryer vent cleaning have to do with preventing fire. This can be a big motivator and a very valid reason to get your dryer vents cleaned periodically. But a fire is not going to start in your dryer vent. If you think about it, there is no ignition source. That spark that turns into a fire. But if you look inside your dryer, well most everything you need to start a big blaze is right there and ready to go. Thus the reason I not only promote cleaning your dryer vents, but to also clean your dryer as well.
All the air that exits your home through the dryer ducting comes from inside your home. As I tell people, dryers make great vacuum cleaners because they are constantly moving air through them. Much of it simply passes through getting heated, collecting moisture and lint, then finally ending outside. Some of it pulls lint, dust and other debris (pet hair) into the dryer as it begins this journey. Most of this rarely makes it into the ducting because it enters the dryer cavity from so many different locations.
The dryer will generally have many places for air to enter the cavity such as louvers along the back and especially every gap between metal panels. This quickly slows the airflow down and allows the debris to begin to pile up like new fallen snow on everything inside the dryer. It is this material that poses the greatest opportunity for a fire to start because it will burn, there is plenty of oxygen, and every dryer has a heat source. Put that fire triangle together and you have the potential for a fire.
Fortunately, dryers are designed the enclose a fire should one occur and if you have it connected to a property designed venting system, you could even have a fire and never even know. If the system is working as designed, the fire simply burns itself out. But if lint is building up inside the dryer cavity and/or the dryer ducting, this creates a situation where a fire can start and simply smolder. Hopefully it goes out, but sometimes that is simply the beginning.
So the moral of this story is to ensure your home and family safety, the dryer cavity should be cleaned as well as the dryer ducting as part of an ongoing home maintenance plan. Anything less is simply playing with fire.