With a 16t/42t ratio, or even when I road with the stock 15t/43t ratios, I have a general shift pattern if I am not in a hurry. 1st - 2nd ~ 20 mph; 2nd - 3rd ~ 30 mph; 3rd - 4th ~ 40 mph; 4th - 5th ~ 45 - 50 mph.
I very rarely go past 25 mph in 1st gear, although I suppose you can.
When I am at different speed limits I generally use the following gears:
25 mph = 3rd
35 mph = 4th
45 mph = 5th
I may occasionally downshift if I need to accelerate quickly, but this usually keeps me in a range I can roll on throttle for easy driving.
I don't thrash my KLR, even though I know guys that do, and I don't have to add oil between changes very often, and I get better than 55 mpg averaging around 58 over the last 90,000 miles.
BUT... that's just my preference, ride the way you like and you will be happy.