I spent ages trying to find this link in my last post, couldn't find it so gave up (was bookmarked on other lappy which isn't with me at the moment). And then I accidentally stumbled across it just now. Life's like that isn't it?
Anyhow,
you won't be sorry you read this- highly informative. David Knisley is a well known observer and he has put the UHC filters and OIII filters thru their paces on a list of planetary nebs, detailing which filters is best for each object and why.
Please, do yourself a favor and read it.
http://www.raycash.org/filters.htm