just make sure that you CUT any level that seems overpowering. For instance, if u are mixing 2 songs with powerful bass, cut the bass on the incoming track, then "transfer" basslines in the middle of the mix by turning down one track while simultaneously turning up the other.
I use this method for bass, mids and highs, depending on what I want to do. It'z all up to you, and there are no set guidelines. It all depends on the sound that u want.
Just make sure that the red dots on your level meters dont go too much in the red. This is known as "volume control". If u do, then the sound will be really distorted during playback, and the mix will be noticably too loud.