I think it all depends on the way you are studying. If you are using MGMAT study guides, then you would probably not have access to OTHER questions for each chapter, so hitting the OG questions timed (w/o much practice) may not be the best option. If you can get a good grip on the topic at hand (through some other material) then the OG practice should be timed. I personally think that we end up wasting quite a bit of OG questions if we move straight from STUDYING/GUIDE to OG without going through an intermediary non Official (but similar) set of questions.

For Verbal, it becomes a bit easy because most strategy guides prepare us well for the type of questions that are tested, so you can move straight from your STUDY GUIDE to the OG timed.