sam21fire

I don't think there is an "ideal" route. If you go South of the NF (roughly parallel to the 210 fwy) you'll be dealing with urban traffic until you pass through the San Bernardino area. After that you'll have a stretch where you'll be forced to ride the shoulder of I-10 (not fun) unless you go farther south and go through Moreno Valley/San Jacinto area. Either way you'll have to either climb up Hwy 62 to Yucca Valley area (several tough grades) or access the park off I-10 east of Indio. East of Indio you're limited to I-10, and there's a pretty steep climb out of Indio.

If you go north of the NF there's less choice of roads and will either have pick your way around to avoid the more heavily trafficed roads (like Hwy 138) or take your chances on the busy routes. Once you get east of Victorville area the route choices get very thin. East of Lucerne Valley you'll be able to drop right into Yucca Valley etc and be able to 'enjoy' Hwy 62 or one of the side routes.

The north route is pretty warm this time of year all the way to JT. Southern route gets seriously hot east of Banning and there's essentially no services (water, food) east of Indio.

Good luck!
Sam