Well they dont do % till first 6 months. May be some small ones might. However, right now there are too less jobs and too many people so its not worth barganing for. Get the job.

You can always throw them out once you have your H1 and a couple of pay stubs.  They are not allowed to keep your paper anyways and if they do just tell them that you are going out for stamping and they will be forced to give you the papers.

have fun and good luck