if there is any chance that dust, hair, dirt, or anything else could get in the paste then you are best off cleaning both the cpu and heatsink thoroughly with rubbing alcohol or something (i use oxy pads!!)
anyways i've taken heatsinks off and put them back on when i got lazy and never really noticed any problems
if the paste is kinda dry tho then that is a definite sign to reaaply it
good luck