Hemlocks and maples could be pruned any time of the year, for sure.
Oaks probably, too.
Don't know for sure about magnolias, but my friend who has about 5 or 6 of them prune them in late March-early April.

Magnolias can bleed, so they are best are best pruned in mid-summer to avoid that risk, while also giving them enough time to heal before winter. I usually prune my magnolias around late-June.

Thank you for your responses. I hadn't heard anything back, so I checked out the internet over the weekend. Most of the info I found was from other areas of the country, but this is what I found.

Oaks should not be pruned April thru October to avoid the spread of Oak Wilt.

Maples, Birch and other bleeders (Magnolia?) can be pruned in mid summer (though the sap doesn't actually hurt the tree).

Somewhere else I read the magnolias and other ornamental trees and shrubs that flower before June 1 (including azaleas and Rhododendrons) should be pruned immediately after flowering.

This leaves a question about the Magnolia (ours is a Star). It's flowering now but the leaves come out after it flowers. Should it be pruned right after flowering or in late June? We're not concerned about sap, so I'm thinking it doesn't really matter. Another question comes up because this is an old Magnolia and its a pretty big top shoot we want to take off. I was reading that you should only prune Magnolias when they are young - they can have problems healing.

The Hemlocks seem to be a question too. Some info from Virginia says that the best time is April thru July. Some other info says its best in the dormant season.

Re: Magnolia.
I ask my friend why he prunes his in March-April thus losing some of the Spring blooms.
Answer was that in that time they are leafless and he clearly see where and how much to prune. If pruned after the bloom when in a leaf he loses the perspective. Also, if pruned in a dormant/semi-dormant stage sap helps to heal wounds.
FWIW.
Re: Hemlocks.
I prune mine whenever I feel like or have an opportunity.
No ill effect ever was noticed.

The folks over on the trees forum probably have answered this a dozen times, so you might want to do a search there if you still have questions.

In my own garden, I don't really prune trees at all, unless there is an unruly, crossed or damaged limb, in which case I prune it when I see it.

"The best time to prune is when the saw is sharp." (My favorite garden related quote, no idea where it's from)