Bad solder joints on a board are often the result of a bad board component drawing too much current and/or generating too much heat. This heats and re-heats the joint, causing it to crack.
It is best to replace a board if you see cracked solder joints. I tried solder them for the first year or so. Then I started getting callbacks for the exact same thing. Lo and behold, the solder was cracked again!! Not to mention, many boards were burn beyond a solder repair.