That's preposterous. The PH matters for the mash. The extract was obtained by a mashing process with proper PH and the sugars are already extracted. I can't see how PH matters later for extract. If it did matter than it would be mentioned all over the place and I've never seen this till your post. BTW, for the record there is NO guarantee bottled water such as spring water will be in a perfect PH for beer brewing. Bottled water can be all over the PH map and also be worse in contaminates than tap water.
Rev.
I don`t want to turn this into a pissing contest but it DOES make a difference. The PH can be thrown off by excess minerals which WILL effect the taste and hop utilization. I am talking from my own experience as well as others. True the PH of the wort will drive the flavor of the beer. A high alkaline beer will taste more bitter than an acidic beer. Also true the PH is more important to the mash however that PH carries over to the beer making process, mash to sparge to boil to bottle. When you make a can of condensed chicken noodle soup do you make it with water out of the ocean? NO!! because Campbells has added the flavorings already and if you used sea water it would have too much salt. Take a bitter beer, it will have an alkaline PH, add some salt which is neutral, and it will work to lower the PH making the beer LESS bitter. There is tons of info on PH and how it effect the taste of beer, during mash, sparge and boil. Add 3 table spoons of gypsum to your next batch of beer during the boil and tell me it doesn`t taste like ass. Bottled water can vary however it will be more neutral in more cases than your tap water making it better for extract in MOST cases. I would suggest some research....
Brew Strong Water Episode 4 (all 4 episodes have a lot of info regarding water for all grain and extract but I didn`t want to go through them all for quotes)
http://s125483039.onlinehome.us/archive/bs_water4_051809.mp3
note: Jamil's comment about gypsum at 33:00 and the part on extract brewing at 38:00 roughly.
http://www.winning-homebrew.com/extract-brewing.html
"If your water has a lot of chlorine you can boil it to remove the chlorine. Higher levels of chlorine will cause chlorophenols to be produced in your beer resulting in a strong band-aid flavor. Adding 1 campden tablet (available at all winemaking suppliers) to your brewing water will remove the chlorine and chloramines and will treat up to 20 gallons of brewing water. Adding bottled spring water is a good but expensive option. Use distilled water for dilution to correct the mineral composition of your water. You can use various types of water in extract brewing since it won't affect the mash pH or extraction of tannins during the sparge. Always be aware that the manufacturer of the extract used water that was full of minerals and salts. These were concentrated during the drying process (for DME), and in the process of removing water in LME. These minerals are still in the extract. If you have water that is high in carbonates, bicarbonates, or any other minerals, you may be at extreme levels after using your own tap water. It's best to get a report so you will know exactly what your water has in it, and use distilled, RO, or deionized water when your water is out of specs."
http://www.winning-homebrew.com/brewing-water.html
"For extract brewers, you don't have to worry about this. The extract will have all the brewing water minerals from the location that the extract was made. If you are having problems with some off flavors, or can't seem to make certain types of beer by adding your own water to extracts, try using distilled water. If you are adding mineral salts to your water when using extracts, you may be over doing it since these minerals are already in the extract. Call the manufacturer to find out the water profile of the brewery that made the extract. You can then make adjustments from there."
