If there's traffic, even light traffic, travelling at well above or below the posted limit introduces a marked discontinuity in smooth traffic flow. Anything that requires the majority of other drivers to make an unplanned large correction in speed or course is, by definition, a danger. To what degree is dependent on traffic density, road type, conditions, etc.. There's also the element of vehicle size. A tractor trailer driving 65 in a 75 is easily spotted from a distance. It gives other drivers time to anticipate and plan (if they aren't too busy texting). A small coupe or sedan is easily lost behind the silhouette of preceding traffic, increasing the likelyhood of panic-response detection and evasion.
If the road is deserted and you are your own 85th percentile, drive whatever speed suites your fancy. Otherwise don't be surprised when drivers trying to blend in with the flow get a little miffed when they have to dodge you.
As to speed-related economy, it's a given. Air resistance increases as a square of speed. Ten mph might not seem like much but when you consider it's 15.4% increase in speed from 65 to 75 the difference in economy-killing drag is significant.
Brad