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I believe that I read in the owner's manual that you should not shift into 6th until the car is doing at least 50 mph. Unfortunately, since I don't have the manual in front of me right now I can't confirm what page it was on.
My rule of thumb is to shift into 6th around 55/60 and shift back into 5th if my speed drops below 50/55, depending on traffic. Of course I also do a lot of skip shifting from 2nd to 5th, and 3rd or 4th to 6th to preserve gas mileage after a quick squirt of power in traffic.
Actually, what you remembered was the bit in the manual that said for best fuel economy you should shift into sixth at 50 which means you can go into sixth before 50. Of course, the easy answer is just to try it.