Is it OK to change root's shell?
No! root's shell has to be /sbin/sh, the default setting, or another
shell that resides on the root filesystem (ie - /sbin/csh). The /usr
filesystem isn't mounted in single-user mode, so changing root's shell
to /usr/bin/ksh, or any other shell not on the root filesystem, can
potentially cause you serious grief.
A nice compromise is to add something like the following at the very end
# if /usr/ is mounted, change our shell to ksh...
/usr/bin/true 2>/dev/null && exec /usr/bin/ksh