Dbltree is right... it depends. It depends on several things. How tall and/or thick is the grass that will be burned? The bark on oak trees can withstand fire pretty well relative to most species, but if the grass is higher than tree itself, etc, then it may be destructive.
I burned an area of my flattened brome CRP a couple of years ago now and the cedars survived it better than I wanted them to. Some even turned orange for awhile and then came back. (They are gone now, but it was a brush hog that got 'em, not the fire.)
A switchgrass fire could be a whole different animal, much more fuel than wimpy brome.