It is best to place them on planets on the front lines, and on planets that are along the path that ships will normally take when traveling through a system. They only resupply ships that are in the same sector (orbiting the planet) as the depot.
My strategy is a bit the same as theirs. It defers in that:
I place them:
Planets I build a shipyard on always get one.
Reason: When you retro fit a ship it has no supplys. As such, it can only move one space till it gets reuplyed.
Barring that, I put it on a smaller planet
If neither of the above cases apply:
I place then as far apart as possible unless there is a planet along the "default flight path"