Yes, that's exactly what I do .... the developers are clever. The fire stations are only 35(?) coins, no matter how many you have. But the points keep rising. So, you get to a point where it is better to buy your new FS with coins, and use your credits to buy other things instead.
As you say, I don't buy coins until the specials come about.
and to expand on that point -wait until the devs offer station builds at 1/2 coin cost, then build like crazy - then expand those when you can, and turn off the others until ready to expand