Wayne,
> As far as I know, the reason for non-parallel walls in studios has to do with reflections, not standing waves. <
Exactly. It's common in professional control rooms for the side walls to angle outward by 30 to 35 degrees or so, which deflects all first reflections toward the back of the room. Non-parallel walls are neither good nor bad from the perspective of bass frequencies. And you can still determine "good" dimensions whether the walls are parallel or not.
That said, the biggest mistake I see people make is building a room with the corners cut off at an angle. Whether the room is parallel or not, whatever you do please leave the corners as corners! Otherwise you've just removed the single best place to put bass traps.
--Ethan