I'm heading up to San Gorgonio Peak tomorrow. I've mentioned to people that in one year of off-and-on trail running in San Diego County, I have (not surprisingly) seen more snakes than in ten years of much more frequent trail running in the Bay Area. Up there, the received wisdom is that if you're above maybe 7,000', you're out of rattlesnake territory. That's in the hills right around Tahoe, or on the way up to Tuolumne Meadows in Yosemite (but not down in the valley.)
But, latitude makes a difference, and I don't want to get complacent about putting my hands where I can't see them above 7,000' in SoCal mountains. Several online sources about the altitude limits of rattlers repeated numbers around 6,500 or 7,000', but then I found an old publication with attributions and named eyewitness reporters, (Rattlesnakes, L.M. Klauber, 1972) which says: "...there is no doubt that [the Southern Pacific Rattlesnake] does reach an altitude well over 10,000 feet in the San Jacinto and San Bernardino mountains of southern California. Joseph Ewan reported a southern Pacific rattlesnake at no to exceed 20 feet from the peak of Mount san Jacinto, elevation 10,805 feet." He goes on to give multiple eyewitness reports of rattlers at these and higher elevations, up to 15,000' in Mexico on Orizaba.
Whether or not these are outliers, they're possible. In nature there are few absolutes. From this weighty philosophical principle we can conclude: regardless of elevation, no putting handies and feeties in little crackies.