If you don't get the hole perpendicular it will lift one side of the nylon wing bolt's head each time it's tightened down. This does two things: it will deform the balsa, or, if a plywood plate is in the wing, will eventually weaken the bolt to the point where the head seperates . . . followed shortly thereafter by the wing.

It's kind of hard to "eyeball" the angle freehand. I have a set of the Higley drill guides and they do help a lot in keeping the holes perpendicular to the surface.