It's always a good idea, for any file system, to keep a bit of free space on it. YMMV, but I do think that 10% is a bit low, so I'd go for a bigger SD card, just to be sure and to cope with people tendency to fill up the free space.
This is clearly opiniated and I insist that in the described situation, with a 64GB card, everything will probably go well, but you never know :-)