It all comes down to what does the owner's manual say. If the owner's manual says regular will cause no harm (that's what my ES300 manual says), that's fine with me.
As long as you understand the issues, and if you're doing no harm, go for it.
But if the manual doesn't explicitly say "does no harm" or some such thing for lower octane, I'd stick with what the manual does recommend.
There's probably a middle ground, where using lower octane fuel temporarily is acceptable; if that's the case, the manual should say so.