do I need US visa when I come back from Canada?

I have h1B visa and just got approval of I140 this month. I want go to Canada for a meeting next month(one week). I wonder whether I need US visa when I come back from Canada. If I do need US visa, how many chance I can get it in a day in Canada? Some of friends warned me that it is not safe becase I applied I140. But I didn't submit 485, I can not apply AP. Any suggestion and idea?
Thanks a lot in advance.
Mike