Well...this is what astronomers had presumed, and taught their students for decades, then during the 1990's the true picture began to emerge. After studies of hundreds of planetary nebulae, astronomers concluded that not only are ring-like nebula very very rare, but other shapes predominated. If you viewed some of these other shapes diferently they would indeed look ring-like!!! Here is a recent quote from the Hubble Space Telescope after investigators intensively studied 'Messier 57' in the above image:
The pictures reveal that the "Ring" is actually a cylinder of gas seen almost end-on. Such elongated shapes are common among other planetary nebulae, because thick disks of gas and dust form a waist around a dying star. This "waist" slows down the expansion of material ejected by the doomed object. The easiest escape route for this cast-off material is above and below the star. This photo reveals dark, elongated clumps of material embedded in the gas at the edge of the nebula; the dying central star is floating in a blue haze of hot gas.