All yeasts I've come across work well within the apple's range of pH.
Adjusting the pH serves two purposes. First, a lower pH helps protect cider from infection, if you have a dangerously high pH, you should definitely consider lowering it before fermentation, or increase the amount of SO2 to compensate. Here's a good table showing what I mean http://www.cider.org.uk/frameset.htm
. Second, adjusting the pH will alter the flavor of the cider, IMO cider lacking in acidity will taste bland, thin, and somewhat swampy. Final pH really depends on the juice used, post fermentation, but prior to bottling, I like to play with the acidity and see what level brings out the most apple flavor, too much though can cover the fruity notes with tartness. Let the apple guide you to the proper acidity, commonly 3.3-3.7 for American ciders, which tend to be more tart than their EU counterparts.