Absolutely. NEVER remove/replace a heat sink unless you also replace the thermal compound.
^ The reason for this is that the thermal paste is now junk. When you pull a heatsink off, you disturb the paste, causing air bubbles and debris to settle in. This creates pockets and builds up heat. It's not good on PC's and especially not good on laptops. If that is what you did, take the laptop back apart and re apply the thermal paste.