Thats personal preference... I like the lower rpm's myself to save on gas, but no harm will come of running at a higher rpm.
But why would you want to? uses more gas...louder..more vibration.. etc... if you need to accelerate quickly, just downshift, only takes a sec.
I run in 6'th going 25 and 6'th going 70 on the highway, just because it saves on gas keeping the rpm;s low, and unles im climbing a huge hill, i dont need to downshift.
Now with the 15t front sprocket installed... I find myself using 5'th gear mroe for lower speeds than 6'th, keeps the rpm low and has mroe torque.