Well...a slightly high ph (over 7.2ish, I believe) or slightly low ph (6.2ish, I think, lol) can stress the roots, making nutrient uptake more difficult, as it tries to deal with the imbalance. (results in nute lock-out)

As time rolls on, the ph in peat based soil mixes tend to go south, but unless re-using your soil it should be ok for a season. I don't usually have problems, but I do flush regularly.