Yeah, you can spot out "high" / "medium" / "low" frequency locations everywhere in Arkham / expansion boards. For instance, in Innsmouth you have only two cards opening gates in Y'han'thlei, while Devil Reef is certainly a much more active place. So, you're right, a good part of the strategy is to choose where to close / where to seal and when. For example, you have only two Mythos opening gates at the Silver Twilight Lodge: if you don't have enough clues to seal, but you need to keep the gate number low, you can choose to close the STL without risking too much. On the other hand, you won't close Independence Square unless you can seal it. The DOR mechanism was created especially considering this! Skilled players tended to seal high freq as soon as possible, having new Mythos opening gates there simply "bouncing" on the already existing seals. Now it's a little more complicated, because the bounce actually triggers the DOR track, so you have to be more carefull about some choices to do!
Anyway, enjoy Innsmouth (a magnificent expansion!) and if you have any other doubt or wanna talk about strategy, feel free to post! There is always someone watching over this forum