Good question! I'll be interested to read other responses because I'm not sure either!
To date, I have just been following Beersmith and adding water at the suggested tempetrature.
I suppose however that since the grain and MT are already in the 150's, your sparge water wouldn't drop all that much anyway - maybe down to the low 160's?
I took a bunch of extra temperature readings last brew day to get a better handle on the process- but didn;t check teh temp in teh MT after the 168F sparge had been in their for a bit.
I'll try to check that next time! I suppose it is not as critical as the mash temp anyway, and from what I have read the main thing is not to sparge too hot, which could then extract tannins from the husks and give rise to off flavours..
Brian.