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I do use an OIII filter for teasing out small planetary nebs. For the really small ones it is the only wy to pick them out even at medium power.
For the bigger and brighter ones they are best without a filter.
Big faint ones (the Helix for example) also benefit if seeing or transparency is a bit poor.
I also have a nebula filter, really improves contrast on the bright nebulae (Tarantula and Eta Car notably) and dims the stars in the neb so enabling better views of the nebulosity, but certainly not essential. For tracking down the faint emmision and reflection nebs, non filtered seems to work better for me usually.
Malcolm
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