If both linear tracks are available, use audio-track one for main sound and audiotrack two for background sound on VHS and S-VHS. Audio-track one is on the inside of the tape and less prone to tape damage. (The positions are reversed with some formats.)
If the equipment offers FM audio, the sound is generally recorded on the FM tracks and the linear tracks at the same time. If not, consider carefully which to use, as the FM sound is interlinked with the video, which can cause problems when audio-dubbing or editing.