It depends. I have rifle sights on mine, handy when I'm forced to use slugs and also forces me to aim versus pointing. Pointing gets you shooting over the targets. If you go Open Class, a nice Aimpoint works.
This is what I use for 3-gun and tactical matches. An 870 from a 3-gunner I bought had a set of these on one of the barrels and I liked it so much when I bought my 930 for 3-gun I put these on also. Makes the slug shots easier than just bead also.
This is a very good question, but not a very easy one to answer well. Traditionally shotguns are used primarily on moving targets that are hit with a string of shot. A bead front sight with the shooters eye being the de facto rear sight is adequate for rabbits and pheasants etc.
I have a FNH that has a fiber optic front sight and a folding rifle (sort of a buckhorn) rear sight. With slugs the rifle sight is helpful, for everything else I keep it folded down. I've tried shooting trap with this gun and it is a poor choice, a proper trap gun is worth 10 more birds out of a 25 run string. However, for 3-gun there are relatively few flying birds and generally these are flippers that fly strait up and then down. They can be shot with rifle type sights if one will wait until they "stop" at the top of their flight. For most of the shots you will typically be shooting stationary clay birds and knock down steel, both of which are shot much like a rifle, but with a "huge bullet". Almost any sights get this job done, but as mentioned above a red dot is hard to beat for speed, because they allow both eyes to be open and picking up the targets is faster.
I'll be watching this thread as this is no where near covered. I'll be interested to see what the real shot gunners have to say.
90%+ of people run a vent rib in 3 run. Many have a small flip up rear. 80% of your SG shots are with bird on plates or clays so thats what you set your gun up for and practice a little with slugs for those 50yd slug targets. You can make rifle type sights work, aperature sights are a no go as they are slow.