Remember that any relative, even the most well-meaning and reliable one, can occasionally tell you something that is incorrect. Verify everything Grandma (or Uncle Herman, etc.) tells you, using other sources as much as possible. Well-intentioned informants can be incorrect and the very process of verifying what they tell you may cause you to locate information you never dreamed of.
People tell me that "I didn't look at that record because I already knew what was on it." You don't know what is on it until you actually look at it. You may think you know and you may be correct. But there's always the chance that you were wrong.