Killing the chip will probably not happen at sub-1.55v if the chip doesn't run too hot (which is the max you'd ever want to use anyway because more yields pretty much no extra overclocking). The only thing you risk when using 1.35v is degradation in maximum overclock and a lower overclock at a specific vcore. I'd say 1.35v is pretty safe and if you encounter any degradation just up the vcore by a tad or lower the clock until it regains stability. For 6 months use I'd suggest to not hold back too much