The water in the tank expands as it heats, and a bit is forced out -- one reason the relief valve is there, and the bit of release is OK. I read a procedure somewhere that went like: after the hot water tank is full and before heating water, turn the water pressure off (pump or city spigot), open a hot water tap and then open the relief valve to drain a bit, thus creating an air pocket in the tank that will prevent dribbles when heating water. I've been too forgetful to try this myself yet, however.
Bruce