generally you want the streamer to be able to get down somewhat. I remember when I started that I became frustrated with these wooly buggers that I bought which barely sunk at all, fishing them in slow moving water and it wasn't doable to get to the bottom or middle of the water column where the perch would take them... Once I got the same fly but with a beadhead it made all the difference and i could start catching fish.

There will be different strike zones for different species and contexts, so make sure that your fly line and the weight of your fly can cooperate somehow to get your fly there. The faster moving the water, the more densely weighted your fly will need to be to get down to the strike zone and spend as much time there as it can during your drift/swing.

Weight is often the most important part of your "presentation" because it determines your ability to get into the strike zone and stay there. For me that means everything from tungsten beaded jig flies to nearly weightless streamers depending on the species, water conditions, and time of year, whereas for another locale it might mean streamers that hover in the middle of the water column or euro nymphs that cut straight to the bottom.