When is it important to handle reflections in pcb design?
This is the quick answer.

1) If the signal is backterminated and/or the amplitude you need requires that you have termination (PECL, video, CML), you have to terminate it.

2) If the rise/fall time of your signal is comparable with the time it takes your signal to transit the path, you have to terminate it.

3) If you have AC coupling, somebody has to provide a DC path to ground on the other side of the coupling capacitor.  Such as, the termination.  If it's not inside the part, you have to provide it.

4) If it's slow (compared to the path length), DC coupled, and the right amplitude at low frequency, you might be able to get away without anything special.