When is it safe to enter the Sierra? This really depends on the year and how much experience you have on snow and in the mountains. The most generally accepted date to leave Kennedy Meadows is on June 15. I left on June 10 and did fine, although the snow was excessive at times. After leaving Crab Tree meadows, the jumping off point for climbing Mount Whitney, the PCT through the Sierras is a a sequence of passes, separated by valleys. Usually, snow would obscure the trail for the last 2-5 miles before the pass and for 2-5 miles after the pass. Sometimes the snow was nothing more than an annoyance for route finding. Othertimes, there was snow hidden in gullies and in cols that made traverses dangerous or difficult. In the morning the snow would be hard, but usually manageable, except before 9 or so. In the afternoon (around 2 or 3), the snow would be soft enough that postholing (when each step drops you to your knee or worse in the snow) became common.
I sense that I won't truly understand the conditions and how to approach them the safest way possible until I'm actually at the kick-off day (ADZPCTKO) and can talk to former thru-hikers in person.
I'm guessing that the problem with snow before 9 am is that it's icy. I don't see why my Kahtoola microspike traction devices wouldn't help with that. I've walked with them on frozen solid icy trails before, and they provide superb traction, biting into the ice with a loud crackling sound. It seems like it would be a wise idea to use those and get in more hours of morning hiking than to submit myself to hours of postholing in the afternoon.
I am also concerned by stream crossings, which are best attempted early in the morning when flow is lowest. But if there are too many streams to cross, I suppose I will inevitably end up having to do afternoon crossings every once in a while.
Finally, I'm a bit scared at the thought of having to go straight up steep snowy passes like Forrester Pass, and doing any other potentially dangerous things. These fears are normal for PCT thru-hikers, I'm sure.
Since I'm beginning on April 24 and foresee keeping a pretty good pace, even in the early sections (maybe 20 mile/day on average), I could end up at Kennedy Meadows as soon as June 1st. I might try to slow myself down and delay my Sierra crossing by deliberately spending more time in towns in the public libraries, or spending more time hiking along the PCT with relatives and friends (I grew up in Southern California). Maybe someone would take me down into Orange County for old times' sake (never been back since age 21 - 10 years ago). Or, I could just get to Kennedy Meadows as soon as possible and hang out in the southern Sierras or Lone Pine area until the snow mostly melts.