Why should I use DC coupling when I'm finding the frequency response of a circuit?
Because AC coupling is a highpass filter.  Using AC coupling will therefore distort the frequency response at lower frequencies.  Since you don't know the specifications of the high pass filter (and since they may also depend in part on the impedances of the circuit under test,) you want to avoid using AC coupling since you have no way to compensate for it in your measurements.  DC coupling doesn't put any kinds of filter between the device under test and your oscilloscope.

Put simply, if you use AC coupling, then your measurements will also include a measurement of the high pass filter in the oscilloscope.