My main SSD is 160GB and would never fill it up to the point where only 20 percent was left. I have three SSD's, main 160, another 160, and an 80Gb as well. I don't see why there would be a need to leave 20% unavailable.
Plus, I wouldn't do that even with a 300GB SSD. If one sees a need for more space, one can buy another HDD/SSD and put large files like Pictures, Movies, Music, etc. on that extra drive. Of course, this is my approach and others may think or do differently.
Thanks for your comment.
So your simple answer to the question of the thread title seems to be: "No". Or did I misunderstand something?
By the way: As long as I have my 2 Intel Postville SSD'S I have always created partitions by utilising the whole SSD size leaving no space unpartitioned. On the other hand I took care, that there was always more than 20% free within each partition.
Until now I didn't run ito any problem or performance breakdown.