Any temperature below the thermal throttling temperature is a safe temperature and that doesn't happen until 105C on a Core i5-3570K. If this was hurting your CPU, why did Intel increase the thermal throttling temperature from 98C to 105C when they went from Sandy Bridge to Ivy Bridge? People worry way too much about their CPU temperature when there is no reason to do this. Intel CPUs have always done a great job of thermal management.
As for core temps being slightly different, this is usually just random sensor error. These temperature sensors only purpose is to control thermal throttling and thermal shutdown. Intel didn't need to use 100% accurate space shuttle quality sensors for this so they didn't. Providing enthusiasts with 100% accurate core temperatures was never the design goal. These sensor are accurate to +/- 5C and that's at the calibration point. Below that where most people run their CPUs the amount of error can be greater and the amount of variation core to core will also be greater unless all 4 cores are running the exact same code.
Intel designs their CPUs so they can run reliably, 24/7, in poorly maintained servers with CPU coolers packed full of dust. Some people treat CPUs like they were delicate old ladies. An Intel CPU can take more abuse than you can imagine and do this while being completely stable. I wish all computer parts were built to the same standard.
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