Hi Mark,
I would not go with Eyepieces for this or that objects, but choose your set from an exit pupil perspective.
There are PN's (quite a lot of them) which need very high magnifications (small EP, short focal length EP) to reveal there nature as a disc for example. Opposite, there are many very big ones which need lowest magnifications (big EP, long focal length EP). The same for Galaxies and basically for all other DS objects.
The 3-2mm EP rule of thump which is often mentioned is a common misunderstanding imho. It just applies to a certain range of objects.
For a good range I would go with this EP's (do not know what you already have):
6mm EP, 3mm EP, 1,5mm EP, 1mm EP and 0,7mm EP
The more you go to high magnifications, the more the steps should be smaller.
For "very good skies" I mean very dark and or very good seeing there is room for 7mm EP and 0,5mm or even 0,3mm EP. The optics has to be up to the tasks also in this case.
To get a good overview to this topic I highly recommend the TeleVue eyepiece calculator on their homepage. I do not say you have to buy TeleVue Eyepieces. ? I like them a lot, though.
You get there a very good overview for all focal lengths and exit pupils in your scope. Just put in the aperture and focal length of your scope. There is much more data there to get an overview what field of view etc. you get with the certain TeleVue EP's. But the data of other manufactures are close so this helps, too.
And try to test every EP at your own scope if possible before you buy. Eyepiece preferences are a very individual.
cs,
Uli