If I remove the heatsink from my processor temporarily, then later put it back without changing the heatsink or CPU, do I need to reapply thermal paste?
The thermal paste is there to provide good heat conductivity between the CPU and the heatsink. If you separate the two, it is best for you to reapply some thermal paste (VERY thin layer) in order to maintain good thermal contact between the two.
I usually prefer to clean both the heat sink and the CPU with a Q-Tip and some alcohol before reapplying thermal paste… And I have never experienced any problems doing so but I can't really recommend doing so in case either your heat sink or CPU does not like alcohol.
I basically fried my CPU back in Dec 08 and needed to buy a new one due to the fact that, I didn't replace the thermal compound. It was something that I greatly regret and whenever someone now says, "no thermal compound is needed", I would laugh at them and their fried CPU if they actually did replace the heat sink without the thermal compound. It just isn't worth taking the risk! I did and where did it get me?