The only way to be certain if the value it finds for subwoofer distance is to measure how well the sub integrates with the other speakers, particularly the mains and center channel. If your sub is 5ft away from your listening position and your mains are 15ft away, yet the receiver thinks the speakers are 15ft away (correct) and the sub is (20ft) away, then the receiver is likely wrong unless the sub itself is adding additional delay due to DSP processing, or a cascaded LPF from the sub. Thus its recommended you defeat the LPF of the subwoofer or at least dial it much higher than the LPf in the receiver.
Don't blindly trust that the receiver is setting the sub distance correctly. Let your ears, and measurements (if possible) determine the validity of the setting.