Low frequencies, specifically sub-bass frequencies (80Hz >), are uni-directional, meaning it is extremely difficult to audibly pinpoint the location of them within the stereo-field. Naturally, they feel wider than a single, high frequency signal. This is why a subwoofer is not always centred within a studio, as it is not crucial to the response.
Conversely, if this sub band of frequencies are actively working the stereo field via panning or other stereo techniques, it can cause problems in various types of rooms and monitoring systems. Take, for instance, a night club…if I pan my bass hard left, the audience among the left hand side of the room will experience a lack of bassline. That rings true, if the club is setup in an optimal way with a stereo system. More often than not clubs are designed with mono systems to make up for the movement of low frequencies in an attempt to create presence at every frequency in every part of the room.
When a stereo signal is summed to mono in this situation, loss in the signal may occur as you are combining the relative phase of both the left and right speakers.
With these concepts in mind, you will be far better equipped with what to pan and what to avoid. Like levelling in step one, deciding on focal points throughout a song, in terms of instrumentation, will further assist in the right panning decision. Pans are static, but can certainly be active too. Tackle active panning after statically panning and placing instruments in the stereo field.
Depending upon the genre of music being created, panning should abide by genre-specific guidelines.
For most electronic music, having instrumentation equally panned creates a stereo, yet centralized mix, ideal for live venue applications. If I decide to pan a guitar lead slightly off centre, I will typically even out the lop-sided mix by panning an equally as important instrument to the other side to the same extent, for instance, maybe a chord line. This way, one side is never favoured, yet you create a stereo image while maintaining the centralized focal point of a mix.
Focal point instruments should never be occupying the same stereo space, or else frequency masking and a lack of clarity will prevail.
Basslines should be kept centred below a nominal frequency cutoff, such as 80Hz. This frequency is genre-dependant, too.
Instruments that are few and far between can work the stereo field in a far more extreme way to add contrast to a primarily centralized, stereo mix. This too applies to ambience or other tracks that are relatively sub conscious.
As panning is complete, the stereo field is not finalized in this stage. Stereo effects are the last line of defence and extra width can be maintained in this stage also. That being said, you should not solely rely on time-based effects to achieve stereo width, just as the loudness of instruments is not finalized after levelling, compression will help achieve a more solidified position as time-based effects will help achieve a more solidified stereo image.
Although, like music in general, there are never hard rules, guidelines can help make the right decisions when stereo imaging. Exercise techniques and test out ideas as this step is one of the most subjective stages of a mix.
Mono sounds have tendency to mask other if not moved off the centre-line or the same stereo-line, while stereo sounds are naturally wide and panning these will merely shift the wide image.