See the link in my sig about tire pressure for more discussion than you'll ever want to read about it. For best FE, it is most likely that you should inflate them as high as is comfortable and handles well (but not beyond the tire's maximum).
I just found the other day in a Neon's owners manual, that the manufacturer said for sustained high speed driving the tires should be inflated to sidewall max not door sticker max.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
Buried deep in many manuals and publications is the recommendation for increased pressure for more demanding conditions -- speed, load, road condition, tire condition, etc. Tires work because of the air, and they work better with more of it than the usual default automaker recommendation.
As far as I can tell, the default recommendation by the automaker is made by the marketing department to sell cars with a softer ride, limited by the legal department (who consults the engineers for the bare minimum) to avoid blowouts. Remember the Ford/Firestone debacle? They either forgot to consult with the legal department, or the lawyers forgot to ask the engineers...
I recommend filling them up as high as you can, but not over the sidewall's maximum rating, while not creating increased wear down the center of the tire. If it starts to wear down the center, drop it by a couple psi. But also abide by theholycow's advice about handling. If higher pressure makes the vehicle squirly, back it down.
Well I have some Toyo Spectrum rated for 44psi. I pumped my tires (was between 22 to 26 psi :S) and they offered less rolling ressistance. The handling is better, but the confort is a litle bit affected (on bad road), so my go back to 40. But the FE is much more better, and I learned that wathing tire pressure is important, even critical, even if you drive a VX.
The winter tires are on the way to be install and I think checking pressure in cold temperature is even more important, I remember a thread talking about the effect of the cold on the tires pressure, will do a search.
Safety and comfort are very important. Safety, for obvious reasons. Comfort, because you can't drive safely or efficiently when you're uncomfortable...discomfort breaks the patience necessary for that.
Winter tires may need different pressure. You'll almost certainly need to adjust it differently for winter conditions than normal conditions.
The temperature itself isn't a problem, but the change in temperature is. If you check the pressure when it's 70 degrees, you can expect it to be less when it's 20 degrees.