surely you mean it can cause lock-out? Which it doesn't, you're fine in DWC up to 6.2... although I'm not saying this is good... it is doable. The plants will absorb nutrients better in the 5.2-5.8 range. In fact, there is evidennce now to suggest that even 5.8 might be too high, and out of the optimum range to uptake nutrients.

In DWC your roots will rot if you allow light onto them, or the res' is too low... or too warm. Plants prefer a colder res, as this helps in the uptake of nutrients too. in fact the temp' of the res is more important than the temp' around the plant...