Impedance matching: is coaxial cable Z important?
If the time delay though the cable is small relative to the wavelength of the highest frequency you are transmitting then termination is not important.

Be aware that if you have a pulse rather than a sine wave at 5MHz for example there maybe harmonic components that are much higher than 5Mhz and that may cause problems.

If for example you are sending a digital signal over a coax to be used as the clock into a register at the receiving end then although the fundamental may be low in frequency you may get reflections that could cause double clocking if the cable is not terminated.

Assuming the cable is 1 meter long with a velocity factor of ~0.6 the two way delay will be:

2/(3*10**8 * 0.6) = 11ns. This is equivalent to 89MHz.

If we want this to be 10 times the highest frequency we shouldn't have any frequency components > 8.9MHz.  The 5MHz figure you quote meets this.