In the last decade or so, most NOBO PCT thru-hikers have started at the end of April or early May. This is fairly logical on a typical year since most hikers make it to Kennedy Meadows in about 6 weeks - allowing for the snowpack in the High Sierra to consolidate. However, there are a number of factors or advantages to starting “early.”
(1) If your not in great shape at the start, it might not be prudent to start off at a 20mpd pace. Starting out too fast can result in a lot of physical problems.
(2) If you get injured or just enjoy taking zero days (and who doesn’t) like a day or two soaking in Warner hot springs, its very nice to have extra time and not feel the pressure of having to move on every day.
(3) The desert is considerably cooler through April and early May.
(4) If you start before the Kickoff party, you will find the small stores well stocked and you won’t have the sometimes annoying feel of a mass migration - businesses would enjoy the thru-hikers being spread out as well.
(5) Its can be enjoyable to hang out in Kennedy Meadows a few days - eating at Grumpy Bear and the store to ‘fatten’ up a bit before the high sierra, where it is very hard to carry enough food to cover the calorie loss over the long stretches. Also, there tends to be a few parties, including Meadow Ed’s big b-day bash. 
(6) If you would like to enjoy a few days on the PCT without seeing anyone, then starting well before or well after the Kickoff (often too Hot) will give you a much better chance at having this experience.
In '97 I started @ Campo on April 25th - this was over a week “early” back then, due to the fact that most hikers almost religiously followed what Ray Jardine had written in his PCT Hikers Handbook. (the earliest starting date, for the “5-month itinerary”, was May 2nd) There was a lot of corn pasta in the hiker boxes that year…
In '05 I started @ Campo on April 8th. I took my time to Kennedy Meadow, arriving at the end of May with the intention of “flippin” to Manning once i reached Mt. Whitney. This year there was an extremely heavy snowpack in the Sierra and an extremely light snowpack in the Pacific Northwest, so it made the ‘flip-flop’ thru-hike very attractive, especially since i had hiked thru the Sierra in heavy snow in '97. The flip this year allowed for an unusually long thru-hike: i finished near Mt. Whitney Oct. 8th (summitting Whitney and finishing the JMT the next day). The High Sierra typically has balmy, dry conditions until mid October, unlike the North Cascades, which can get a bit wet or snow-bound towards the end of September & early October.
In any case, i hope this info is usefull for people planning a PCT thru-hike. If any other alumni PCT thru-hikers have any input, feel free to jump in with some other ideas!
Happy Trails, freebird
freebird