A visual inspection for leaks is very important. If there are any leaks in the piping, they need to be repaired right away to avoid introducing fresh water into the system. Any time new water is introduced, there are also more impurities introduced into the system. These will be deposited in the boiler's heat exchanger. This will eventually  cause a leak and also cause a premature heat exchanger failure. The reason is simple. The impurities that are brought in, will settle out of the water as it is heated. This will leave a scale deposit that will act as a insulator on the water side of the heat exchanger, causing the heat exchange to over heat and warp. The water valve should be in the closed position when the boiler is in operation. The closed valve will prevent any leaks going undetected, because the system was able to maintain pressure by refilling itself via an open valve.