They definitely belong on any colony that you have a stake on, otherwise you wouldn't even know if something hostile is in orbit. As for DSTS-only listening posts, the question is how much minerals and industry you need to invest to cover something that matters, a question heavily dependent on system geography. Form an RP perspective I don't put garrisons on as they're supposed to be secret (usually), stationing tens of thousands of soldiers exponentially increases the chance that the location would become known. gameplay-wise, ground forces also have a signature that can be detected. But if your secrecy fails and a competitor discovers it they can swoop in and take over some potentially expensive hardware, so it's a risk.