There'* a big debate going on over on ClubGP on this right now. Some say it does, some say not. I have seen no compelling documentation one way or the other.
Although I have not done a perfect A / B comparison (that is, no other changes at all) between 180 / 195 in my 1998 LeSabre, I believe the 180 T-stat is dinging me for about 2 mpg in my mixed city/highway driving (i.e., daily commuting), while allowing me to advance timing (with tuner) a few degrees. I'm getting a consistent 17-18 mpg, where I think I should be getting 19-20. I believe this mpg loss (if I can confirm it) may be because the fuel is not atomizing as well when the engine is not heating up as much.
I don't believe the 180 T-stat is having any measureable impact on mpg'* (or timing allowed) on long, cruising highway trips. There I get 32-35 mpg, depending on how packed the car is (far less of course when we have six people in the car, the car-topper, are towing the popup, and are heading to Big Bear / Tahoe, etc. )
I plan to go back to a 195 degree T-stat in the very near future, and see if my mpg'* go up. If so, I'm sticking with 195 and getting a bigger aux trans cooler. Gas prices are such that a few degrees of timing advance isn't worth a maybe 10% loss in mpg on the driving I do >75% of the time