Hi Jock:
I did Toronto to Los Angeles and back on my ST1300 two years ago. I did a very southern route to get there - more or less down to New Orleans, then west on smaller roads that paralleled I-10. On the way back, I went north up to Salt Lake City, then more or less paralleled I-80 coming home. I enjoyed the trip very much. There are enormous cultural differences between the southern and northern halves of the USA.
It is extraordinarily boring to ride on the American Interstates, so, unless I was 'transiting' an area, I avoided the interstates. There is a very fine collection of secondary roads in the USA called
America's Byways, and these are the roads I recommend you ride. Naturally, it will take longer to get from YYZ to LAX on these roads (8 or 10 days each way makes for a comfortable trip), but you will really get the opportunity to see the country and get a sense of local history and culture.
I also highly recommend maps produced by a company called
Butler Maps. They make maps specifically aimed a touring motorcyclists, including a 'Route 66' mapset. They are not cheap, but for sure they are worth the money - using those maps, you'll be able to plan a thoroughly interesting and pleasant route.
Regards,
Michael, Toronto