Your wort temp is what's important, not the grain bed. The starch and enzymes have to interact in order to convert to sugars. the enzymes are in solution in the wort and if you're getting the wort too hot in order to heat up the grain bed, you risk going above the temperature at which the enzymes can operate. If your wort is 168 or higher for very long, conversion will stop, regardless of the temperature of the grain itself.
If you're recirculating and controlling temp, the grain will eventually be as warm as the wort but it won't matter much during the conversion process. My system allows me to measure and control the wort temp within a little over a degree of the desired mash temp. I've found that I've gotten the best efficiency and the best wort quality by not worrying at all about what the grain bed is doing and keeping the wort just so and raising it as needed to the next step temp, letting the grain bed catch up or not.
I start the recirc before I add the grain. By getting the strike water to the proper temp, or just slightly higher and milling directly into the tun while the strike water circulates, the grain picks up heat a little at a time and the RIMS tube keeps adding heat as necessary to hold the exact temp until all the grain is in the tun. From there I usually let it settle in and then rest a little with recirc off while the malt absorbs some liquid. Then I can restart the recirc and start raising to the next temp and adding more liquor ( I almost always dough-in very thick at around 122 or so and then raise to the main conversion temperature).