ZPG and NPG are bad. You want positive population growth. Population growth is good for the economy and is a resource for nation that make productive use (in a positive way) of their people. Strain on resources has to do with inefficiencies, political and economic corruption, overconsumption of resources by wealthy (including middle class), etc.
I've said this before, and I'll say it again: the planet and countries can sustainably support many, many more people than it does now. The problem is with wealth (particularly land) distribution and human nature, not with population. There are enough resources on the planet to last an indefinite amount of time, but not if everyone consumes at the rate of the average middle class American or greater.