On the low income housing, I can understand your point to some extent.
Most of these folks will have a poor credit score, and the prospect needs to be looked at in a more non-traditional way.
I just feel that a decision based only on looks and the cleanliness of a car might not be the best business habits to forge. I am not saying that these are not factors to consider, but more steps should be taken before a final decision is made.
If you really get a "gut" feeling that this is not a good person, I feel that there are better ways to deal with it than by putting yourself in a potentially dangerous situation. I see nothing good happening by "exciting" a prospect and/or refusing to show them the home. If this is really a bad person, they could do you some physical harm, or worse!
I can also see some legal issues that could happen. Even though the person might not be in a protected class, it does not mean that they will not make an attempt to sue you. While it might be frivolous, it still takes time and money to defend yourself.
My advice is to at least go through the motions of your normal protocol. That way, if the person does try to sue for something frivolous, you have all of your records showing that they were treated the same as every other tenant prospect.
Important Point:
Michael, It is great advice to take a look at the criminal records. I do not think enough landlords take this into account, and it is something every landlord should add to their screening process.