Mileage will vary on this one. Apple uses various different thermal goodies on its machines. The black or grey ones which tend to be smallish and used almost entirely on CPUs (as opposed to GPUs) and other chips. These tend to be in a bit of a state after you remove the heatsink and I personally always replace them. Even if I only replace them with a layer of paste. The blue ones which tend to feel greasy to the touch, tend to be bigger and used more often with other chips as well as some CPUs. These tend to be OK as long as the heatsink wasn't too tight, as they compress quite easily.
In short, always change the grey/black ones, if the blue ones look OK, use them again.