Are Replacement Words For Swear Words a Bad Idea?

Teaching your kids right from wrong can be difficult. I like to think I have a good outlook on what is wrong and what is right but explaining the "rules of the world" can be difficult. My three year old has asked me many times if "heck" is a naughty word. He has also asked me why we read about God but we can't say "Oh my..." These rules can be hard to explain to young children and sometimes I don't know if I have the right answer. Is heck a bad word?