Nextel nights start at 9pm on the National Free Incoming 300 plan, and all Nextels plans. You can now get them to start at 7pm, an extra 2 hours, but in my opnion it's not worth what they charge you for it. I think it's an extra $5 or $10, not sure. Ask around. Someone else might know if you're interested.
I too have the "National Free Incoming 300" plan. I like it. It's all about outgoing minutes, I agree. I know it's rare that I will ever reach my 300 min. outgoing limit, that's why I got it. I use more of the free incoming and nights and weekends minutes myself.
So if you can guage in any way how many outgoing minutes you have used before, or would use. I don't know if you can tell that from your T-mobile billing statements. But once you know that, go from there in choosing the 300, 500, 800, or 1000 outgoing plans.