The 2000 model's engine really is sluggish under 4500 - 5000 RPM. Though many times while cruising in town I am around 5500 - 6000 RPM. Your RPM effects several things: The loudness of the bike, the gas mileage, the available torque, the ability to get out of a trouble spot (through acceleration) and several other factors. Remember, the bike doesn't redline until after 14,000 RPM, so you got plenty of RPM room to play with.
I'd say for in town cruising find the balance that works best for you. In town I am never in top gear, usually 3-4 gear is fine. Higher limit roads maybe 5th, I don't hit 6th unless I am on the freeway or a long straightaway with at least a 50 MPH limit. If it's late at night I'll be in a higher gear than normal to keep the engine noise down. So it realy depends on the how/what/why's of riding when picking an RPM that's best.
At 45 MPH I'd say the 5000-5500 range is fine. Downshift a gear and ride there if you think you'll need sudden power.
-Flash