A block heater should be a good thing.
I haven't installed one (because I do mostly fairly long one hour commute trips) but I've read enough favorable posts by hypermilers who use them. They're especially good if medium or short length trips are what you do most - because a high percentage of your drive time + distance is spent heating up the engine.
I definitely recommend a grill block. Even with a block heater, driving through the cold winter air steadily washes cold air over the engine, causing you to continually spend fuel replacing that lost heat.
And be sure the thermostat is working properly. My ScanGauge shows engine temp much more precisely than the simple dash gauge. I know the '90 Civic won't support a ScanGauge but my point is, just because the gauge needle eventually gets up to the center don't assume the t'stat is working right.
Pics of my grill block. My car does warm up much quicker with the grill block in. I used foam pipe insulation. Some of it is hot-glued into position.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.