Deciduous Magnolias in general don't really need much in the way of pruning, unless it is necessary to shape or control size. You can do light pruning now, or you could wait until they have flowered. If you don't want to sacrifice blooms, I'd wait until later. Magnolia's don't typically tend to get water sprouts, (large, thick stems that shoot straight up, similar to what deciduous fruit trees such as Apples are notorious for. Some Magnolia soulangeana cultivars may do this, and if they are grafted, and the shoots are coming from below the graft, you will want to remove them.

Do you know what type of Magnolia you have, or can you post a photo? This would help with giving you better advice...

I would prune whenever. If you're in Oakland, I'm surprised they aren't already in bloom. I've already seen a number of them near Temescal and over near UC Berkeley campus that get started around Christmas and peak late this month. Bloom is a month later inland, but your's is sure to get rolling really soon.