Don't worry about it. If there is a problem with your machine, Apple will replace it. If you can, make sure you have it a few weeks before you really need it, so you have time to get it swapped just in case.
In terms of Rev. A, every product Apple releases could be classed as Rev. A, because it will have something new in it, or something replaced. I mean, what would be the point of Apple releasing a machine that was exactly the same as the old ones? Yes, Apple does fix problems that are found with their products, but it's not like Apple announces after a month or two that their "Rev. B" macs are now officially flawless. The longer you wait will be beneficial, but if you wait for Apple to iron out all the bugs, a new computer will be being released that very next tuesday.
Buy when you need, but weight it towards just after Apple releases a new product, because then you will have time to replace any duds that you get, on the unlikely possibility that you do get a dud, of course.