The performance of the filesystem often degrades a lot when it becomes full. Fragmentation suddenly increase rapidly and the result can be highly increased disk activity possibly even causing premature disk failure. In a striped RAID filesystem the risk of damage is even greater.
If you really need to fill the filesystem, write to it sequentially until full. You can now read from it fine. But if you try to update or change anything the space-time continuum starts bending and the age of the drives increase rapidly at the same time as the writes becomes slower and slower. Or something like that...
At around 80% you should start thinking about expanding the filesystem and/or removing files.
Yes and no. It is being used to make sure the HDD is working fine and to avoid fragmentation. However it is only needed if you update/delete/write to the drive. If you just read from it you can fill it to 99.9%.
With RAID you have already "wasted" one or more drives on the altar of parity, so what is 10-20% more? 
If the filesystem is really big and you store small files then 5% might be OK. If the filesystem is small and you store big files then 20% might be bad already. The percentages are just a guide. It is when the performance actually start to decline you have a problem. You could run some benchmark now and then on the drive to check the performance. And see when problems start to show.
Also the inner most tracks are often used last, and that will slow down everything. The read head fly speed is slower there.
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