I'm not sure what sort of map you tend to play but these improvements can be especially useful on continents maps. The landmasses tend to be elongated shapes reaching from pole to pole. By carefully distributing the improvements you can ensure that any city is close to either of your palaces, or Versailles (should you be able to build it) even when having conquered the entire continent. This can, in turn, dramatically reduce maintenance cost.
Keep in mind though that forbidden palace and Versailles cannot be rebuilt once established, while the initial palace can. As such, you should aim for an as ideal as possible placement for the former two while the latter is sort of a 'wildcard'. Ideally you'll want to distribute them as evenly as possible across your available landmass.
On that note: don't be afraid to move your palace around. I've noticed many players are very reluctant to do so but the cost is not THAT high either. At the same time, the initial city is fequently almost predestined to be a GP farm. Having the palace in such a city means both the bureaucracy and extra commerce are more or less lost.
Above all, distance from capital can make a substantial difference. Even, say, a mere 5 less gold spent each turn due to better palace placement really adds up in the long run.
One last point: don't bend yourself backwards to obtain Versailles. It's a good wonder, no doubt, but expensive as well. Unless you're playing an industrious leader and/or have access to marble I personally wouldn't consider the investment, especially since it requires Divine Right which is quite oddly placed in the tech tree (i.e. it's a sidetrack tech)
