The character of a mix’s foreground elements shape the overall character of the mix.
Sounds obvious, doesn’t it? Clearly, the sounds that are loudest and most prominent in a mix will contribute the most to the overall sound of the mix. Likewise, the sounds that are quietest won’t contribute much.
In practice, what this means to you will depend on what stage of production you’re involved in – performing, recording, mixing, etc.
If you’re recording a song, you should pay careful attention to the sound of the main elements of the song. For example, if you’re aiming for a crunchy lo-fi sound, you’ll get most of the way there just by tastefully treating the vocals and drums (assuming a regular pop-style song). If you’re careful (or bold), you could even process the audio on its way in. Similarly, if you want a slick clean sound, you should focus your efforts on making your front end as hi-fi as possible – including instruments, mics, preamps and recording interfaces. You can probably get away with a bit of grunge in the background or incidental instruments, so long as you get the foreground instruments spot-on.
The same goes for mixing. If the foreground tracks already have a strong character you should go with it. Work with it, not against it. Similarly, if the foreground tracks are quite raw, you have some freedom to shape the sound of the mix – even if the background or incidental sounds have some more character.
Keeping this in mind, you’ll realise that you shouldn’t spend too much time on establishing the character of the background instruments. Every little bit helps, but remember to focus your time on getting the foreground instruments exactly how you want them. Don’t waste time getting carried away tweaking a sound to perfection, only to bury it under layers of more generic-sounding tracks.
Focus your time on where it will make the most impact.