When in doubt, check the owner's manual. The book's tablesays upshift to 6th at 65mph. Leave 6th on the way back down at 60mph.
Hmm ... the '08s must be different from the '07s ... my owner's manual says that the upshift (acceleration) is done "Fifth to sixth" at 55 mph (85kph) ... table 21 on page 78. The downshift (deceleration) is done from Sixth to fifth at 50mph.
For what it's worth, 55 seems to slow to me unless you're going at that speed for a distance. I won't say it "lugs" as much as it just seems to lope along. At a constant 55+ speed I think you'd be okay, but if I'm trying to accelerate to entera freeway or other high speed trafficway (60 and above) -- or if I'm going to pass a slower vehicle -- then I'll usually shift at 60 to 65 instead of 55, but that's on harder acceleration.
My dealer told me not to hit 6th until I was at 65 mph or I would lug it and that would be bad for the engine. I followed their advise for a while, but occasionally I lose count when I am coming up the gears. I have hit 6th at 55-60 mph, and have had no problems with lugging, and that is running stock.
About 60 for me, if I'm just loafing . It's just plain gutless at that speed and rpm, but downshift into 5th and it comes to life. 70 seems to be optimum,as it powers up nice and smootly. Ron