I've pitched at both ends of the extremes. I pitched my Arrogant Bastard clone at about 18 hours and my nut brown at three days and both turned out great. I've been told (several times) that one of the primary functions of a starter is to confirm your yeast is healthy, if you have krausen it is ok to pitch. Just like everything else in brewing (or so it seems to me) it's all a matter of personal preference. I've read that some people like to pitch at full krausen (18-24 hours) and some like to pitch after everything has settled down (24-48 hours).