rs485 redundant termination resistor
The termination is required because of the characteristic impedance of the wire used for the bus ...typically a twisted pair. 
Putting multiple termination resistors on a short cable will load down your signal. 

Read this application note.

If you have in and out connectors on your boards, then you should supply a Terminator that the user plugs into each end of the physical bus. That way you do not need terminators on each board.

Alternatively you could put a resistor on each board and a switch, which would be set (turn the resistor on) on the leftmost and rightmost board on the bus. This is easy to describe to a user.