Hard to say. Attachments can give bonus and negatives effects. It's possible.
I'd really have to say it's all up to the player's preference. The real point in customization in this game is to make the weapon you want to use 'yours'. The way you want it. The point of customizing in this game wasn't to improve the statistics of guns to the point of them being game braking, but to improve the statistics that you see fit that applies to your play style. Splash Damage is trying their hardest to balance absolutely everything, and so far they seem to be doing a terrific job.
http://www.youtube.com/watch?v=daFJGJ2tCDM (Skip to 1:34 to view what I am talking about)
Watch this, it goes over customization. While they won't go into detail about the effects of everything, watch how adding some attachments affect the stats. Now, this video is from a while ago, so it is subject to change.
For example, putting a scope on your rifle or other weapon does not increase accuracy or even the RANGE However, I'm sure they'll give you a bit of zoom, although there aren't any extremely large areas (that I know of) for them to be absolutely necessary. Suppressors have no effect on the weapons as well. Notice when the grip is added. A very MINUTE amount of 'stability' is added. I'd imagine stability to be very related to the term recoil, so say it only decreases the effects of muzzle climb by a very small amount, but increases the time it takes to equip the weapon. So it is really based on how you want to use the weapon and what your play style is. The only point I agree on are the use of larger capacity
magazines. They are definitely useful. The only downside to those are the use of more ammunition. You are more likely to use more bullets and possibly run out. The only way to replenish them during combat is to have a soldier give you more. But the EXP system should keep them crapping ammo all the time. Weapons themselves have been designed to not so much be better than others in its class, but have tactical advantages for certain situations. Hell, I will probably find myself using a bare weapon just because I can. I love iron sights myself. (I also noticed adjusted iron sights, so I'm very excited if they still exist). Either way I'm sure you'll have to unlock attatchments.
Once again, Splash Damage is trying very hard to balance everything so everyone can play the way they want, depending on what is needed during objectives.
Hope this helped.