Do I need biasing resistors on the +/- lines for RS485?
If you need to have a guaranteed level when no transmitter is enabled then you  need biasing resistors (assuming a receiver that does not have a built-in offset, as some Maxim parts have). 

Not having bias resistors means that you could have false received data. 

Typically you'd have one resistor from A to +5 and another resistor from B to GND and a third resistor from A to B. The values are chosen so that the voltage level such that the logic level is defined (200mV or greater) and the desired differential termination resistance is presented.