While many of the PNs appear round, or elliptical as is the case for the Ring Nebula, many of them are decidedly not round:
Courtesy NASA, ESA, and the Hubble Heritage team. One can see why the term "Cosmic Butterflies" has been used for these objects: see the nice book by Sun Kwok for example.
Courtesy NASA, ESA, and the Hubble Heritage team.
Courtesy Bruce Balick, NASA, and ESA.
Both these last two PNs must have some mechanism that is shaping the gas into the long, somewhat cylindrical structures that we observe.
This is an image of the PN Abell 39, taken at the 3.5 meter WIYN telescope near Kitt Peak, Arizona, by George Jacoby, Gary Ferland, and Kirk Korista. This is a narrow-band image in the 5007 Angstrom emission line. I include it here just to show that indeed some of these PNs appear to be close to spherical in shape.
One thing to note is that these images are 2-D projections of the 3-D shapes of the objects. The Ring Nebula is actually thought to be more or less a cylindrical nebula viewed close to the axis, rather than an ellipsoidal nebula. The images can therefore be misleading, and we need to study the detailed kinematics to deduce the actual shapes.