shifting, shifting, just keep shifting...
Shift when you feel comfortable, and the bike sounds "right".
FWIW: When I ride 55-65, on a twisty road, I'd be in 5th so that the motor has more snap/power available. Putting around in 6th is OK too.
Steady interstate 65mph: Try to avoid interstates myself, but 6th gear would be appropriate and fewer vibes.
Parking lot zigzag: 1st if it's really slow- you do NOT want to stall this tall bike!
Under 6500 on tach: There's no "need" to run the bike harder than that, but it sure can be fun!!!!!
2006 V-Strom 650 (red)