Not always, it depends on the cost of whatever is giving the unit
FNP. Basically, against most weapons,
FNP causes the squad to take 33% fewer casualties against any given round of shooting. Thus, it could be argued that
FNP is only worth it when the squad costs more than 3 times as much as whatever is giving
FNP. However, this is the minimum theoretical value where taking the upgrade is worth it. As long as the unit size isn't not maxed out, you have to remember that the opportunity cost of taking
FNP is X marines (I don't have the chaos codex, so I don't know the costs of the Icon of Excess and a Noise Marine), which means you get less shooting, fewer opportunities to get upgraded weapons, and fewer casualties before you have to take a leadership test. Plus, you have to factor in that the Icon of Excess can be sniped out of the unit, not reliably but it can happen, negating the bonus. Thus, I would not recommend taking an Icon of Excess UNLESS the unit costs more than 5 or 6 times the cost of the Icon (assuming the Icon costs about 25 points, which feels right for squad-wide
FNP, I wouldn't suggest taking one for squads costing less than 125 points, excluding the icon).