I've never found myself in a situation where I thought "Gosh, I wish I had an ND filter with me".
ND grads, now that's another matter. I have a 2stop hard and a 1stop hard. I use them both about equally. I find I can get great skies with the 1stop in conjunction with a polariser.
I'm wondering whether a ND soft grad might be useful as well. As I use both the 1 and 2 stop hard grads equally often, I'm thinking I might go for a 1.5 stop soft grad. I think Lee make them.
That's my line of thought. Hope it helps you make your mind up.