Termination of digital lines
It's not the frequency of your signal that matters - its rise time of your signal that matters. 1Hz and 100Mhz signals with the same rise time, will suffer the same problems because they have the same rise time.

A good rule of thumb is to aim for 20% the rise time. 

If you have a signal with a rise time of 1ns, then the maximum time delay you can have without termination is 1ns * 0.2 = 0.2ns. In FR4, the speed of a signal is ~ 6inches/nsec, so you can have a 1.2 inch trace without much issue.

What that also means, is that if you have a trace that is longer than 1.2inches with a signal whose rise time is 1ns, then you need to terminate, otherwise you'll be running into signal integrity issues.