Tenant Screening: What if gut reaction is "no" but credit, etc. checks out?

[Baley Whary]

I've read the ultimate guide to tenant screening and I've also been reading on the forums and another book, and I hear people say, "look at the car the prospective tenant drives" or "get a feel for how they present themselves," but at the same time, the only way to actually disqualify a tenant, without the chance of a discrimination suit, is to disqualify them based on your renting standards (3X income, credit score, background check, etc.). So my question is, what if I just really don't want to rent to someone, based on their attitude, or the trashiness of their car, or whatever? Otherwise, why does everyone say "listen to your gut"? I don't have a specific example, but this is just a question that has come up. I'm super new at this and doing as much research as possible before situations arise. Thanks in advance!