Boiler servicing is recommended yearly by most manufactures and what needs to be done depends on the type of boiler and when it was previously serviced. You will find that a lot of engineers will do a brief safety inspection and may not do a full service as per the manufactures instructions. This maybe OK on a recently installed boiler but as it gets older you will find that it will need to be properly cleaned and checked to maintain efficiency etc. Also such things as the air pressure vessel may need to be replenished and burner and combustion settings will need to be checked and adjusted. With any open flued appliance servicing is even more important as any malfunction could cause products of combustion (fumes) to be spilled into the property and this could result in Carbon Monoxide poisoning.
I carry out full servicing in accordance with the manufactures instructions for the specific boiler and saftey check with flue gas analysis equipment.
It is always useful if you can let me know exactly which boiler you have (make and model) and whether you have the installation and servicing instructions.