Should quad verts be on same plane for subsurf?

I’m a newcomer to modelling and have been playing around with subsurf for a few weeks now. Many subsurf modelling tutorials I read say that I should avoid using triangles and just use quads, which sounds reasonable. But my question is this; for each quad should I strive to keep all vertices aligned on the same plane (I hope the answer is no because it’s very hard to do)?

No unless you want the subsurf to be an exact plane.

BTW: if the plane is straight you can easily make it flat by resizing to zero on the axis perpendicular to that plane. (hitting X,Y or Z once or twice during resizing.)

…then typing 0 (zero) on the numpad (you may need to type N first, but not sure…)

No, the four vertices don’t need to be perfectly coplanar, as you say that is too difficult. But also don’t let your quads get too distorted or become concave quads (v shaped).