Do you always install a septic tank before a reed-bed system?

Yes, for several reasons.  

It traps and settles out solid wastes and objects such as female and male toiletries, paper and clothes which contrary to good practice are often carelessly disposed of down the toilet.  It also prevents these items appearing on the surface of the reed-bed.

It acts as a basic grease trap sufficient for most houses.

It provides Primary Treatment which reduces the amount of work the reed-beds have to do.