September, perhaps?

If we decide to go inland on the return trip, July and August may be a bit... um... toasty.
Bakersfield averages 97 degrees in July and has been as high as 118! (Although that was a hundred years ago.)

Bakersfield weather

Unlike the coast, places like Bakersfield can be 90 degrees at 2:00am. That can be downright miserable. It is considerably cooler in June and September.

There are a couple of other benefits to going in September:

Gareth and I can probably use some extra time to "become one with the bike". That will be important when we're in San Franscico's insane traffic.

Also, it can be difficult to find accomodations along California's central coast during the peak season. (There is about 130 miles of nothing from Carmel to San Luis Obispo.)
It may already be too late to make reservations.

Going after school starts would be a lot easier--particularly if we want a lot of room to vary our schedule (that is, if we want to travel without making reservations).

The central coast is very touristy and they take pride in their ability to extract the entire contents of your wallet (taking a bit of skin with it, if they can)--particularly in upscale places like Carmel.

Here are some examples of room rates along the central coast:

Carmel
San Simeon
San Luis Obispo
Ojai

Perhaps camping deserves a second look.

The weather will be warm and I have a tent that can easily sleep eight--and it can be divided into four rooms, for a little bit of privacy.