Kettle pH is an important factor for reducing tannin extraction and harshness. 5.4 is about as high as you want the overall kettle pH to be. But, there is a lower limit for kettle pH where the finished beer will be tart and hop expression will be reduced. That hop expression issue was just reported by Colin Kaminski yesterday at the NHC in Seattle. I have the feeling that the lower ph limit is subjective, but its probably not lower than about 5.1.