I spend most of my time in the Golden Trout Wilderness, south of Carson-Iceberg Wilderness. I've been hanging there for 4 years, and backpacked there all my life. You will have no problems hanging in the Wilderness, or for that matter anyplace else... As for National Parks, I'm not sure on that one as I try to stay out of the National Parks (ie Yosemite) due to crowds.
What time a year? The last few days we have been getting snow in the Sierra. For the most part, May through September, expect lows in the 40's to 50's, with the occasional 30's. I have seen snow in June. So it's best to have your best insulation if you are cold natured. During the summer a thin ground pad works fine in my Clark North American. There really isn't a big temperature change between 6000 and 10000 feet. Above 10000', it tends to be tree line so plan to stay below that to hang.
The Sierra, during the summer, is very different than the Mountains on the East Coast. It isn't hot, nor humid and very few bugs in comparison... But some places can have a lot of mosquitoes so have deet and a bug net handy.
If you have any more questions feel free to ask... and have a great trip. What part of NC, my mom is from Goldsburo.
Here is a website that has cams all over the Sierra: http://sierracamnetwork.com/index.php
And some information on water quality: http://pweb.jps.net/~prichins/giardia.htm