The following article addresses "Electromigration", or in Intel's terminology, "Processor Degradation" - http://anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3251&p=6
A few simple points:
(1) If you're not going to be using your computer when away or asleep, then shut it down. This saves processor hours, hard drive and fan bearings, dust accumulation and electricity, which means extra beer money.
(2) Configure a BIOS "Profile" for overclock settings, and another for stock settings.
(3) If you don't exceed specifications, then you shouldn't break Intel's 3 year processor degradation curve.
Core i7 specs:
Vcore Max 1.375
Tcase Max (CPU temp) 68c
Tjunction (Core temp) 73c