I think it depends how comfortable you are with the problems in OG12. My preference would be for you to master those problems before going onto the supplement, so if you're still shaky on some of them, spend some time reviewing. If you're only having trouble with the last, say, 20 problems, then don't worry about it that much. But if you're stuck on earlier (i.e., easier) problems, definitely solidify that stuff before moving on.
Also, it's probably a good idea to at least do the easy problems in the quant supplement. Easy is relative of course, but I'd say doing the first 25% shouldn't take terribly long and will give you exposure to more questions. You never know - one of those easy questions might trick you, and having seen it before will mean you won't fall for it on test day.
Jim S. | GMAT Instructor | Veritas Prep
