I am still something of an all-grain noob myself, but one thing I would like to chime in on is that I have done it both ways. My first all grain I actually did 3 sparges of about about 1.5 gallons each. I got 70% but I think I might have gotten a little tannin extraction, because by the time I did the third sparge, the pH of the mash would have been climbing a bit, and the quality of the beer suffered a little bit in my opinion.
On my second batch, I did a mash out with a gallon and a half of 200 degree water (added before taking first runnings) then a single sparge of 4 gallons. This hit my preboil volume perfectly, and gave me exactly 70% again, but this time with no risk of tannin extraction (due to the pH staying low enough). Now I know I left a little sugar in the tun, but I think a higher quality beer is worth it.
I don't know what potentially I might have got for efficiency on the second attempt if there had been another sparge, I'm still experimenting a bit, but 70% isnt bad in my opinion. That being said, you may want to try it both ways and see how it works with your system. You may find that a single sparge makes it simpler and gives you better beer.