what if I don't even get a stamp ?

First thank you so much for your reply.

I've already traveled between us/Canada border several times recently. It seems when I go through airport, I got a stamp with expiration date, but when I travel by car, I don't get anything at the border on my passport, although I can see my most recent I-94 and travel history from the i-94 website.
I noticed I've already had an I-94 card from one of my previous travel (expiration date: Sep 15 2014).
Side note: I'm 485 pending

My question:
If I travel by car before Sept 15 2014, and bring both H1B approved notice, but did NOT get a new expiration date, do I need to come back to the border after Sep 15 2014, just to get a new expiration date ? if so, when should I do it ? anytime after Sep 15 2014 ? Does it matter ?