If you don't have to do, then just don't do.
You can only get 2 entries in 3 months.
Very very possibly you will get background check.
Don't count on your luck. All my chinese friends beside me and the chinese H1 people I met at Toronto consulate on that day, received the background check. I had been waiting for three weeks. Horrible experience! How to keep your employment status during this period is really a headache.
Upon a H1B visa stamp, you will be issued a new I-94 when you return US by crossing the land border. On the back of the new I-94, it clearly states that when you leave US even by crossing the land border, you have to give back the I-94 to Canadian Official.
As to getting a new I-94, you'd better ready to show your 797. So bring with you just in case, although you don't have to (they will check out all your info from their computer database).