150*F to 170*F is the range that John Palmer recommends. According to John, 170*F is the maximum you must not exceed because it can extract undesirable tannins from the grains that will yield off flavors.
Read all of chapter 13
in John Palmer's online book. I found it quite helpful.
When I did my first brew with steeping grains, I put the grains in the water at about 150 and started a 30 minute timer. I continued to heat the water to 165, then shut off the burner and let it sit for the full 30 minutes. Worked great.