Over the years, certain instruments have settled into traditional placements in the stereo field. Bass, kick drum, snare drum, vocals, and low tones tend to be mixed to the center of the stereo panorama, and higher frequencies tend to be spread wider in the stereo field.
Back in the day, there was a very good reason for this: In the days of vinyl records, the needle tended to stay in the groove better if the low end was mixed equally between horizontal and vertical needle movement (left and right). In fact, if the bass was too unbalanced (or too loud) the stylus could literally jump out of the record groove.
But we’re not limited by what’s best for a record needle anymore — unless you’re releasing on vinyl, of course. With digital formats, there’s nothing stopping you from getting wild with the stereo field. You can place any frequency or instrument anywhere you want, to create unique spaces and spatial effects. Kick panned left and bass guitar panned right? Sure, if that’s what the piece of music calls for! The only limit is your imagination!