Good design would dictate that the dropouts should tolerate full (same as for steel) tightening on the axle nuts, especially in back. It's hard enough to get rear wheels so they don't slip under chain load, so it would be poor design to force someone to back off from properly and fully securing his wheels.
That said, I'd shoot for the lowest torque that I felt i could trust to hold my wheels on, and keep the rear from slipping.
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