How should we account for the length of series termination resistors for length matching?
The answer is simple:  Use the same termination on both signals.  

If you are length matching, then timing is important.

Termination can effect rise/fall times which is essentially a timing variation.  Since we have established that timing is important, the easiest way to keep timing on the two signals the same is to make the two signals the same.  This means that they have the same termination, the same trace lengths, the same-ish signal routing, same trace impedance, etc.   

It is possible to have termination on one signal and no termination on the other and still get the same propagation delay.  But to do so is super complex.  You would have to model the driver, the receiver, and everything in between.  You'd look at edge rates and logic voltage thresholds.  You'd look at hysteresis, and pcb trace capacitance, etc.  But in the end, a seasoned professional would just terminate the two signals the same way and save a weeks worth of time.

Another thing to consider:  If timing is important, then the signal speed is high.  If the signal speed is high, then why would you not terminate both signals?  The point is, you probably need to terminate the other signal anyway.