Follow whatever lets you represent the song the best. In songs with vocals, the vocals tend to be really prominent most of the time, so representing the vocals closely usually represents the song fairly well. The vocals also tend to be the most varied out of all the instruments in the song, so following them gives your map some natural variety. This obviously varies between songs, though, so again, follow whatever lets you represent the song the best.
Of course, sometimes another instrument does something more interesting than the one you've been prioritising. In that case, you'll usually want to switch your focus for the interesting part. For example, someone might follow vocals for the most part, but ignore them to map a stream on a drumroll.
Another situation where you might want to switch your focus is when the instrument you're prioritising gets boring. For example, if there's a drawn out vocal or the vocals stop completely but the other instruments keep going normally, you'll usually want to switch focus to the other instruments in that part.
So how do you actually "follow" an instrument?
To put it simply, just make sure most of your active objects (circles and sliderheads) land on sounds from that instrument. Of course, you can mix in other instruments too, as long as it's still easy to tell what the main thing you're following is. For example, it's pretty easy to follow 1/1 and 1/2 vocals while also doing occasional 1/4 for drums. On the other hand, if following both drums and vocals makes your rhythm constant active 1/2, you're better off just following one of the two.
As for also representing the instruments you aren't following, sliderends and repeats are useful for that. A lot of the time, you can follow one instrument with circles and sliders and have the sliderends land on sounds from other instruments.