I was started on the statin when I got my diabetes diagnosis, and at first it was Lipitor (started by a doc at a free clinic where I went because I was uninsured at that time). When my regular doc discovered it, he, knowing our dire financial straits, switched me to the generic lovastatin. I didn't notice any difference at all - I was on Lipitor for about three months. What it HAS done is yank my lipids levels in line - and I DO take it at night.
Whether or not it was having an effect on BG, is more than I can determine, because when I wasn't able to get those levels in line even with increasing dosage of metformin, I started eliminating carbs from my diet until I was seeing the BG numbers I wanted.
That's where I am now - taking lovastatin & metformin both, with 50g-60g of carbs per day, and trying to keep BG between 80 and 130.