I recall seeing somewhere on here that someone monitored their valve clearances whilst trialing different rpm at cruise. They came up with 6500rpm as the point where they saw increased valve wear. How accurate their testing was I have no idea, but it gave me a target to aim for. I try and cruise at 6000rpm now, its a nice comfortable speed where the engine makes good power without reving itself silly.
I find 17/42 good for cruising at 110km/h-ish.