But just think if you had to shoot someone and when he/his estate decides to sue you for whatever stupid reason and how much fun the plantiff's attorney will have in casting you as a paranoid gun nut, just itching to blow someone away:
Depends where you are. In Texas, you are likely to have a jury full of paranoid gun nuts (at least outside of Austin).
Seriously, there is nothing wrong with answering the door with a weapon. You are entitled to defend yourself in your home (in fact, you have no duty to retreat in the home). As UT said, though, make sure you know what you are doing. If you shoot someone without justification, whether you had the gun loaded when you answered the door or whether you got it afterwards, you are going to be in trouble. By the way--a civil suit would be the LEAST of your worries in the case of an unjustified shooting.