Not really. If you're installing a new heatsink on it, then by all means replace the TIM. However, if you're just leaving the same sink on it, only clean it off if you break the contact between IHS and heatsink.
I actually reapplied my thermal paste on my big typhoon with my e6600 when i was cleaning the dust out of my system (plus i probably put too much thermal paste the first time around) but i only noticed 2c cooler at max. Not really worth the effort unless you re getting a new HS or if you broke the contact b/t the HS and TIM like RallyMaster said.