Here is my 2 cents on this. I was on a job years ago and wiring up a resi living room. There were 3 entrances to the room and the builder just wanted 1 switch to the room. I asked him if the HO wanted a 3 way. He said what is the Min. code and he just wanted 1 switch. Couple years later the HO asked me why I didn't put in a 3 way and the answer was that since I was working ( and paid) by the GC, I had to follow his orders. I asked the GC if I could ask the HO about the 3 way but was told I couldn't. The HO said that they wished the 3 way went in.