Same opinion with our pediatrician regarding the MRI...and if they are actually concerned with growth on the brain, the opinion makes no sense. I understand that it is unusual for brain involvement, and if there are no signs of any problems, I would go with whatever you feel comfortable with.
That being said, and always doubting pediatrician/family doctor advice anyway (I won't get into those stories now), I wanted to get the MRI earlier. After all, hemangiomas have their fastest growth early on (first 6 months), so why wait until there might actually be a problem? I explained the situation to a client of mine who happens to be a neurologist. He was very concerned and said we needed the MRI right away (at 2.5 months old). Now, he is a very nervous fellow, so I knew I would get the opinion I sought from him.
Anyway, when the ped heard that a neurologist thought she needed an MRI, we were there within a few days. If you have a concern, use all of your resources to get what you need/want.