Some people wear formally; let's say a suit and a tie and stuff like that while the others ( in my opinion, majority of applicants ) prefer to put on just a T-shirt and jeans.
The thing is this :
No matter what you wear; they just need to be tidy and clean.
I remember that I just had a plain shirt, and my pants were what I was regularly wearing.
Oh! something which might be important; when I was inside the US consulate in Istanbul, that was too hot there, and I had a sweater or something. I couldn't dare to put it off, and I had to tolerate the situation for nearly two hours :))
Relax, whenever you think it's necessary to do somthing, do it!
The funny thing was that I saw some turkish people who had some clothes which seemed to be nightdress!
NOTE : The thing I told was just about what I experienced at the US consulates in Dubai and Istanbul; any comment about any other US consulate or Canadian Embassy is very welcome!