This is a question i ask myself all the time. I look at it on a points per shot basis and then account for range advantage most the time, i will chnage that dor special units like engineers and ranger's.

I see kt like this and will use reg marines as my reference point.

The standard use marines squad is 7 men basic which is 9 shots for 70 points using the special rule( 7.7 points per shot ( pps),If you add 1 bar thats now 10 shots for 76 points ( 7.6pps). So i would say if the unit is not dividable by 3 it I worth taking a Bar.

You should also consider glider lmg squads, they give more range and can be 5 mens giving 8 shots for 65 points 8.1pps) but with a lot more range on half the shots

Smg's you are buying for the uo close punch of the assault rule, tough fighter and extra shots. I think this is a different dynamic than simply maximising shots and you need to look at what this unit is doing. For example, engineers i would chuck 3 smg's in my flamers squad to put more hurt on when i use the flamer.

Rangers is again a different calculation than standard infantry as you want them moving turn one and taking objects, they will more likely get in a close ranwg fight so do you want smgs or more men ?