It depends on the race and the type of weapon. Some races are more warlike and would have full arc coverage or more weapon locations. Other races prefer sheer punch factor so they would have limited weapons arrangements but more powerful guns. The number, shape, and appearance of the emitters would also vary with the design of the ship.
I don't know about anyone else, but all of my non-Feds have modeled weapon points where suitable. The Klingon disruptors I have (aside from the obvious cannons and engine mounted guns) are typically octagonal domes placed at strategic points over the hull. The Romulans and Andromedans are just the same, except I tend to use rounded hemispheres since they have more organic styling. The Cardassians have small reddish boxes across their ships for the phaser emitters.
The Klingons and Romulans don't really have that many barreled weapons, in fact the only obvious ones are on the B'rel. Most Trek weapon emplacements tend to be dome like or not clearly shown for whatever reason. It's a slight irony that people go to great lengths to model in those tiny little phaser domes on TMP Feds, and they're hardly even noticeable. When it comes to the other races though, nobody seems to bother much.
Modeling the weapons does tend to make a difference since it adds that little bit of extra detail, and makes the ship seem somewhat more 'real'. Not to mention it beats having disruptors and phasers come from out of nowhere on a ship. In the end, it just depends on how far you want to go, the polygon limits, and whether you think it would make any difference.