I do not acidify sparge water with lactic or phosphoric, but I am adding salts to the water at the same rate as for mashing and my final runnings are always under 5.6 and usually closer to the mash pH.
The final beers are good, but there is room for improvement (I'm a bit of a perfectionist). The beer that finished at 5.0 was an IPA that took 2nd BOS in a small local comp, but I talked to the judge and he said there was just something about it that was missing. The hops were great, the malt where it should be and it was really good, but there was something, he said, not quite there, but he couldn't pinpoint it. He did use the term delicate describing it (it was a 70+IBU beer). Looking at it delicate and dull could be close to me. I was thinking that it might need a bit more sulfate, but I'm not a huge fan of over the top sulfate.
I am brewing mostly pales, APA and various IPA's, but have done a few others. I use custom profiles when brewing, still trying to dial in the correct levels. The afore mentioned IPA was Ca=50 Mg=10 Na=5 SO4=105 Cl=45 and no bicarbonate. I've since raised the Ca to about 100 SO4 to 225 and Cl to 40. I am thinking of backing off the SO4 a bit though. I also use a touch of Aciduated Malt to lower pH. I use brew'n water and it is almost always .2 pH higher in estimation than actual, but it is very helpful. I measure my pH about 15 minutes into the mash and check after adjustments (usually don't have any) and at the end of the mash as well. I use a pH meter that I calibrate before each use and I lower the temp of the samples to 77 which is what I calibrate at as well.
With all of that said, I'm not sure if this is a mashing issue. My pH's are pretty good at these points, though I could be wrong. I'm wondering what other processes are at work that effect the final pH that I may be doing incorrectly.