I tried just using 1st today, but had to go into 2nd for a while when I was on the motorway. Do not recommend.
 
You shouldn't labour the engine that's for sure. If you drive much below 1500rpm it will probably be less economical than at 1500-2000rpm as when you try to accelerate the engine works too hard. Back in 2005 I bought a new Civic Type R and it came with a one day professional driving course. I was taught:

3rd gear for 30mph
4th gear for 40mph

The idea being it gives the driver more control as the car will be more responsive if you need to accelerate and easier to slow/manoeuvre if you need to stop

Jimbo