"So feel free to fill your drive full of random data just so you can see how fast it defrags for kicks; you will not harm anything, but do not do it on a regular basis unless you want to lower the MTBF of the drive down to mechanical HDD standards."
As pointless as that would be...if you choose to do it for grins, be sure not to completely fill the drive. Leave ~20% free. Defrag moves files around, thats obviously the point. So if you fill the ssd it can't move files using the ssd only (thats if it runs defrag at all, as I'm not sure it will use swap space).