Question: For the 4 jobs that I have worked at, I dressed formal twice, business casual once and more casual once. How important is the dressing code to programming interviews these days? Will the employer or interviewer not take you seriously if you come in casually: plain shirt and khaki pants with casual walking shoes(non-sneakers)?

Answer: 
If you already know what the dress code of the company is, dress consistently. It doesn't hurt to be *slightly* smarter.

Otherwise, the employer will take you more seriously if you phone and ask before you turn up.

If you don't ask and you turn up looking smarter than the dress code, I don't think anyone will think less of you. If you turn up looking less smart, they will.

(If this sounds hard, imagine what it's like to be female and often get the response, "Shirt, no tie.")
My general rule of thumb is dress as though you were meeting a client....b/c you are. Your product is yourself and your skill set.

Unless they tell you that the interview will be 'causal' go full suit and make sure you look neat and professional. If they do say causal I would even suggest going business causal to the interview.