There are a lot of resources available on resupply including this one. Assuming you and your partner have few dietary restrictions (allergies, vegetarian/vegan, etc), you largely can resupply from stores near the trail. This may be a bit restrictive in terms of variety at some points, and will carry with it greater expense in a few others, but ultimately, it does provide some flexibility in that you can purchase what appeals to you at that time, not what sounded good two months ago (in the event of sending supplies ahead).
Off the top of my head, I'd say that the most important points to have a resupply strategy are generally to the north. As you get into Oregon and Washington, there are fewer towns "near the trail" so this necessitates a bit more planning. Please note what I write below is as of 2009 and because of all the changes I'd encourage to check on the current status of these points, especially in light of the recent Post Office closures around the country.
Looking at your list, Warner Springs isn't a bad spot for a resupply. There is a tiny gas station store there where you can get HEET and things like that. (THis is all circa 2009).
Aqua Dulce has a grocery store but if you are going to stop at the Sauffley's, well, it makes sense to get a box there.
Kennedy Meadows - yes send a box there. Be aware if you are using a bounce box that they do not deal with outgoing mail.
Belden - that's a good spot for a resupply box.
Cascade Locks - There is a grocery store in this town. I don't believe it's necessary to resupply here. You can easily to do it at the store.
White Pass - there is a store here, but it's a gas station and thus limited. It does close fairly early so be aware of that. I sent a box here - alternatively, if you can get a hitch down to Packwood, there are bigger stores and lots of places to eat and grab a motel.
Snoqualmie Pass - big gas station and a restaurant. I'd probably send a package here if I didn't live nearby.
Stehekin - Yes, I'd send a package because that would force you to visit Stehekin and partake in the bakery, which by itself was worth a 2,500 mile walk. Be aware of the bus schedule, particularly late in the season.
You might want to think about your Sierra supply strategy as whole. Some elect to hike from Kennedy Meadows without exiting via Kearsarge Pass down to the Onion Valley and the town of Independence. There is a store here, places to eat and a small motel. Much larger stores can be found in Bishop, including an outfitter. There was a bus that would run between the towns as well.
You can send a package to the post office in Bishop, although the store would work as well.
But if you elect to bypass Kearsarge Pass, you can try to make it all the way to Muir Trail Ranch or Vermillion Valley Rance. Both accept boxes for a fee. VVR is generally more suited for hikers and is 174 miles plus a boat ride from Kennedy Meadows. Keep in mind that if you elect to hike up Mt. Whitney, that will generally add a day into your trip.
Twenty five miles or so beyond VVR is Red's Meadow, which has food and some groceries. But you can take a bus from there into Mammoth Lakes and they have big supermarkets there and many restaurant choices. A bit beyond that is Tuolomne Meadows, which also has a Post Office and a store (openings do depend on the snow conditions/time of year.) I had some shoes sent here, but no food. I was desperate for new shoes.
After that you have choices. Many hikers elect to go into South Lake Tahoe. I elected not to go into that town but instead sent a package to the Echo Lake Post Office where I also sent back by bear canister. It's right on the trail. A very good place to send a box, in my estimation.
Beyond that Truckee has a huge store. You are okay there. I'd say Sierra City, even though it has a store, is a place I'd send a box. The reason is the store was largely picked over of food easy to transport on the trail by the time I got there.
For the rest of California I just used what I could buy at stores. I didn't resupply via a mailed box again until I reached Crater Lake. There were a few small lakeside stores where you could buy some food so that wasn't an issue. Beyond that it was the same story - a longer stretch where I carried my food but would augment with snacks or a meal we went hiked near the likeside "resorts". I had a package sent to Big Lake Youth Camp at mile 2002 (and just a mile off the trail). I'd recommend this place, very friendly people and willing to hold your box. I sent warm clothes here and it paid off because it was just about at this time that the rain and cold came.
From there it was White Pass, Snoqualmie Pass and Stehekin, and of course you could augment/buy supplies if you visit the Dinsmore's (just below Steven's Pass).
Please note that it's a good idea to check where fuel for your stove (if you elect to cook food) is available. As I recall, I had to carry extra fuel in much of Northern California and Oregon because it wasn't regularly stocked at the tiny stores.
And as to how to send boxes? For those packages sent to places other than post offices, some places take USPS, some take only UPS. One thing that I found helpful is to resupply from the trail - that is send packages ahead to the next stop or two without stores. This way you could choose things you actually liked while hiking, not what sounded good when you were planning the trip at home.
Have fun on the hike!