I use a MLT instead of BIAB, so I can't tell you for sure, but I think you should keep it pretty close to 170. You have to overshoot on the strike water because the grain is much cooler than the water (presumably room temp), so you lose a lot of heat to that initial mix when you form the grist. But when it comes time to sparging, the grains are very very hot and don't take near as much heat away.
I batch sparge, and Beersmith has me use 168F for my sparge water.