When I do extract with steeping grain recipes, I always just throw the grain bag in the kettle while it is getting up to heat, then remove them when it hits 170.
The reason I do extract recipes (instead of my normal AG) is to save time. I don't want to have to deal with an extra 30 minute step where I could have had the burner firing full bore to get up to a boil.
All my steeped beers have turned out very good, although I cannot comment on whether they would have been better with a half hour of steady 150-155 degree rest.
My thought process comes from my personal decision of AG vs. extract, I know that some people do not have the equipment for AG.