How critical is it for SN65HVD1780DR A/B Differential signal lines to have matched trace length?
9600 bps equates to about 0.104 milliseconds per bit (1/9600). 

The signal speed on a PCB microstrip (external trace) is calculated by:



where Vs is the velocity of the signal, c is the speed of light (11.8 inches per nanosecond), and Ereff is the effective dielectric constant (about 2.92 for FR-4).

The signal speed on a PCB stripline (internal trace) is calculated by:



where again, Vs is the velocity of the signal and c is the speed of light. Er is the dielectric constant of FR-4 which is usually around 4.

If you do the math out, the signal velocity on a microstrip is approximately 6.9 inches per nanosecond and the signal velocity on a stripline is approximately 5.9 inches per nanosecond. If your bit rate is such that each bit takes 0.104 milliseconds (104000 nanoseconds), then you would need a length difference of roughly 717,600 inches (11.3 miles) for a microstrip or 613,600 inches (9.7 miles) for a stripline to present enough delay in one of the differential lines for the receiver to miss an entire bit. You really don't need to worry about length matching on a PCB when dealing with only 9600 bps.

NOTE: Feel free to check my math, I rushed through this on my lunch break.