Hi Glen,
That would probably depend on light pollution in your area, moon phase, bandwidth of your narrowband filter, the length of exposure and brightness of your target.
I use 3nm filters and flats are essential because of heavy light pollution in my area; however, with some bright targets like the Carina nebula and shorter 2-minute exposures I can get away without using flats.
I use one of these and it is gold:
http://www.iceinspace.com.au/forum/s...ad.php?t=49736