I do not believe population expansion should be assisted. The human population has grown on its own without assistance for millions of years and there is no reason to believe it now needs assistance. If anything it is best to let things happen naturally, both population growth and population decreases.
With a sufficient number of natural resources that are essential to support human
life suspect to being able to sustain current population levels, assisting in
population expansion would be irrational. Assisting in the increase in the
number of humans already in existence on a planet that is more gradually
growing to proportions akin to busting at the seams, would certainly not be in
the best interest of humanity, or the survival of the species.
No, population expansion should not always be assisted, because people should not be having children they cannot afford. Each nation in the world has a strong welfare state now. If people who do not have jobs have children, it is a drag on the economy. Populations should be allowed to expand and contract naturally, based on people making good personal decisions for themselves.
The expansion of the population is not always good, nor should it always be encouraged. Things like overpopulation, quality of life, crowding, and depleting natural resources should be considered. The more populations expand the more resources are needed and more areas are used which in some cases, like the rain forest, need to be protected.
The exact opposite is true: people should be encouraged to group themselves closer together in cities. Cities are by far the most efficient ways of housing populations and organizing people, allowing people to travel less far (and, therefore, use less gas/drive less) and have access to more resources around them.