I usually don't go into 6th until 65-70 mph. I don't have a tach but that feels about right (this is the first street bike I have owned that didn't have a tach).
The recommended shift points in my 09's owners manual show going into 6th gear as low as 55 mph. If I'm riding "real" easy, the engine takes it in stride. Normally, I'm rarely in 6th at anything below 60 and am downshifting out of 6th when I drop below that speed. As Minnesota pointed out, 65 to 70 is a safe speed range especially if you're going on feel alone.
My engine feels like it's lugging a bit if I try 6th under 65-70 or so. I try to keep the rpm's somewhere between 2500 and 3000. If I begin to feel some engine vibrations I'll drop down a gear to smooth things out. 09 Low Rider riding solo.