If you'd like to be more scientific about it, try this site:
http://oit.williams.edu/nebulae/browse.cfm
and select the planetary nebula you want to view.
Each nebula has a spectrum showing the emission lines for that nebula.
If the predominant emission is the two O-III lines at 496 and 501nm, then use an O-III filter. The narrower the bandwidth, the better.
If there is significant emission at the 486nm line of H-Beta, then use a UHC filter (or UltraBlock or NPB).
If there is also significant emission at H-Alpha (656nm), then an older UHC filter with the H-Alpha "leak" is preferred, or a newer filter with the H-Alpha broadcast.
Here's a link to a thread on another forum with links to sites that test filters and show which frequencies they transmit:
http://www.cloudynights.com/ubbthrea...t=1&PHPSESSID=