The answer will always be, "it depends." It will depend on what you're trying to do. It will depend on road and traffic conditions.
If it's late at night and there's no one on the road and I'm cruising down a boulevard with all green lights (or no traffic lights), I might very well tool around at 35 mph in 5th gear. But if I'm in city traffic, with cars around me, and I'm going from red light to red light, I might very well be in 3rd gear to take advantage of being in the sweetspot of the powerband (as someone else mentioned) so I can change lanes with ease, or scoot away from another car that I think is charging up behind me.
By the way, an oft-forgotten fact: when GM released one of their recent versions of the Pontiac Trans-Am, Chevy Camaro Z28 and Corvette (maybe five years ago?), the manual transmission was designed to go from 1st gear directly to 4th gear if it felt you were just driving lazily along (ostensibly to save fuel). It would lock you out of 2nd gear, so as you pulled the gearshift down, it would slide right into 4th gear. Too many people complained about it, so it was never used again -- but food for thought.
Bottom line: drive your car, learn what's comfortable for you and your car. If you can, given your relative lack of experience driving stick, try not to drive your car during periods of lots of traffic so you can get a feel for your car, at different speeds, and experiment with what gears you should be in. Frankly, when I learned to drive stick, by that point I had been in so many manual-driven cars that it was natural for me to select the correct gear because I simply had a "feel" for it. You'll develop it, it just takes time behind the wheel.
Good luck, enjoy the new car!