As far as I understand it Metformin doesn't actually effect the food you are eating it with unlike some other medication. It just stops your liver dumping glucose quite so much so doesn't have to be taken with food. However as it frequently causes stomach problems it is advised to take it with food as this can reduce the side effects. I guess it depends on how badly you are effected by it. I rarely take mine with food as I suffer very few side effects, I'm not even on the slow release variant. I tried taking it with meals and half the time forgot about it and ended up taking just before bed anyway so I just changed to taking it last thing.
I would say therefore it is up to you. I doubt waiting until after the scan will make a lot of difference but if you don't suffer the side effects I don't think there s any problem to taking it before.
