0 is black for displacement so 127 is a good middle ground. However, I usually set max tread at 255 and then that gives me the full scale to black for an indentation. Additionally, I usually kick in a little color difference to emphasis the difference in depth.

For coarse treads, like on a tractor, I'll do some low poly vertex modeling and then use Carrara's sub division smoothing.

Dwarg, using displacement mapping does increase poly count, but only when you enable it. So, you can still work with a fairly lightweight model and then click a switch at the end.