The problem though belisarus, is that manpower becomes a problem. As much as possible I try and recruit allied or factional units and use them for garrisons. But if expansion is fast or there have been significant losses or you are playing a faction with a very few starting cities then mercs might be your only option.
Yeah, gold sets the upper limit on your entire army's size, but recruitment rates limit how fast you can build or rebuild units. And you don't really want all those recruitment points from conquered cities go to waste.
But it's probably best to spend them on units that go out and bleed for you, rather than garrisons that sit and draw pay and rarely need replacements once you've built them. I'm going to try accompanying my army with a handful of home-grown spearmen to put in the cities as I conquer them. In addition to saving on gold, that will keep the main force from having to sit in garrison until something can be built locally. The extra spearmen can also do light-duty work such as screening the flanks while all the heavies are tied up in a siege.
But as you say, you can't always do exactly what you want.