It will technically speed up fan death if it is constantly set high but by the time this occurs you'll likely hage upgraded or be thinking about upgrading anyway. Push comes to shove, fans are not that difficult nor expensive to replace.
You could also download MSI Afterburner. This will allow you to set your fan speed for certain temperatures. Obviously 70% is pointless at 30C but then, as you say, at 80C+ you may want it to hit 100% to cool it off quick.
Hope this helps :-)