There is a lot of debate on how, when or even should magnolias be pruned and the consensus is that removal of dead or damaged wood should be the primary goal, otherwise keep your hands off!! Some trees simply do not respond to pruning well and magnolias are in this short list.

Evergreen types wil tolerate pruning better than deciduous forms and young, newly planted trees will tolerate it better than older, more established trees. Pruning is recommended to be done in late summer to early fall as winter pruned trees will bleed (leak sap) extensively and are very slow to recover and sometimes do not. Hard pruning is never recommended, regardless of timing, and no more than 1/4 to 1/3 of the canopy should be thinned out or removed at one time or during the season. The development of lots of suckers or water sprouts will typically indicate a too heavy hand in pruning or improper timing.

I agree with GG. If you mean stellata or soulangeana types, they look best left alone imo. If they're planted in a suitable position they shouldn't need much or any pruning.

Southern magnolia on the other hand has been used many times for topiary or espalier to good effect.

I prune my tree all year long. But I am in east Texas. Warm most of the year.

For that advice to be applicable kitasei would need to have the same species as you Treenutt. We don't now what type she has. As Embothrium points out it depends on the Magnolia. The Spring flowerers wouldn't flower if trimmed year round.

On the other hand, M grandiflora is frequently used as wall espalier over here and is consequently heavily pruned.