I went through most of the JMT (PCT) in June, and so my experience was likely a bit different, partly because I was physically and mentally up-to-speed when I started that stretch, and partly because there's more snow in June, and perhaps less bugs (I saw almost none until after Tuolumne meadows).

A couple of notes regarding the bear can --- you mentioned possibly renting one. This might be a good idea. I own a bear can, and so naturally used that (Bear Vault). Mine's a fine can, but a fellow hiker I was with for part of the JMT portion felt happy to have rented the lighter weight Bearikade. http://www.wild-ideas.net/
OTOH, it's not cheap to rent; you could almost buy a different brand of can for what it costs to rent. I liked my bearvault brand, both for the product itself, and their outstanding customer service. http://www.bearvault.com/

Of course the best option of all is if you have a friend that can loan you one. It's a bit of money to spend and then storage space at home for something that very rarely gets used. Seems to me that there might be a rental program at one or more of the ways in to the JMT, but I'm not sure on that.

One final comment on the bear can, especially if you end up renting one locally (local to the JMT): think ahead of how you plan to carry it. Some people carry it inside the pack, some strap it on the outside. I started with the latter approach and switched to the former, but that was a challenge as I didn't use a large volume backpack. It's also worth thinking through how much food you plan to pack in it, based on estimated caloric needs and where you expect to resupply; the types of food you pack are a factor (how compressible).

The Sierras truly are awesome (not the George Bush type of "awesome", but actually invoking a sense of awe). I definitely want to go back!