Hi Will,

That's a good idea...For what I draw, the construction lines really help a lot to position my curves, so I find if all the object snaps are on
all the time, and the straight snap is also on all the time (unless it occasionally prevents me from doing something),
it's not hard to find the center of anything. So even if a curve isn't planar to the control plane, you can lay down a construction line on the edge of a circle at a quad point, snap to a perpendicular point on the other side and then the mid point of the construction line is your center. I almost never use grid snaps, they annoy me when I forget to shut them off! :-)

-Jesse