Yep, correct pH (for the type of beer you are making) will affect attentuation and extract dramatically. One of the readngs did last year stated that extreme pH's can overide the mash temp and dictate the sugar complex.
Manticle and I have found ourselves championing pH again it seems. Water Chemistry can be made simple or difficult. First, you are better to add some calcium than none at all. Second, calcium is not just for pH manipulation, but is also thermo-protective of malt enzymes, helps trub formation and forms an ionic bridge between yeast to thelp them bind and floculate. Third, if you are trying to match a target water, most of the time you will get close, and close enough is good enough (the Beersmith water tool is pretty good, I've found the Palmer spreadsheet diffcult to use and has elements against what I was taught at Ballarat).
The pH that gives sugars of the highest fermentability and enables maximum sugar production is a pH of 5.2. Most big lagers head straight for this range obviously, but not being one to chase extract, most of my beers sit at 5.4 - 5.6. I want some body in the beer and a higher finishing SG without using a heap of crystal malt for example (finishng SG's are between between 8-14 depending on the beer and mash temp).
Never assume though that your water will always be the same. All matter of forces change the composition (rainfall or lack there of, switch in source, time of day). The water reports you get online are an average, a mate who works for Veola at Waterfall dam commented they manpulate water thoughout the day by menas of co2 acidifcation mostly.
Anyway, no need to panic. Plenty of relaxing can be done with a good homebrew. As mentioned, makng it easy on yourself by adding a small amount of salts and checking as an easy process. It will also give a better reproduciblilty and reliability to your beer by such an easy to do test.
If your still not sure, drop us a line, Manticle also knows a shite ton on this and could probably explain better than me,
Scotty