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: 
Ask
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then wear a suit anyway. or not. and some more characters to meet the minimum.

this ain't rocket surgery
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.