In general terms, I would not disagree with that. But if you choose a
high quality Low Rolling Resistance 185 tire, my experience has been that you will not see a loss of fuel mileage compared to something like a 165 Dunlop Enasave.
The tires I run (Bridgestone Ecopia EP422 Plus) are 185s...but their footprint is almost identical to a 165. Bridgestone improved these tires a few years ago, and one of the things they did was to reduce the contact patch. Here's what a Dunlop Enasave 165 looks like next to an Ecopia EP422 Plus 185...