Yes, you will be required to submit a final transcript once you've graduated. The graduate school your applying to will need to confirm that a bachelor's degree was actually awarded. While I've never heard of a graduate school rescinding an offer of admission based on your performance in your final year, they will rescind their offer if you don't graduate. Also, if you've gone from receiving steady A's and B's in your sophmore and junior years to C's and D's in your final year, they may question your ability to complete advanced coursework...but like I said I have never heard of a graduate school rescinding an offer based on someone's final grades. I have, however, heard of graduate programs requesting transcripts after grades for fall term have been posted on a transcript before making an admission decision. I know "senioritis" can sometime affect someone's performance in their last semester of undergrad, but don't think it's is something that you can blow off either. It could bite you in butt in the future when applying for PhD programs, fellowships, internships, etc.
You are not obligated to accept their offer. However, if you do accept their offer and later change your mind, you need to get the program/grad school to "release you" before you can accept another offer. If the grad school doesn't grant you a release, you're pretty much SOL for a year.