I dislike using things that kill if I can possibly reduce a problem another way.
As you indicate too, I think it is better to try and achieve a natural balance. The trouble is that our annual gardens are in nature the result of disturbance, so in my opinion can never be in perfect balance, all we can do is try and tilt it in our favour. If your conditions favour slugs like mine does, then I find barrier method the most effective - even a small wall can deflect most of the problem. A cut off plant pot or yoghurt pot seems to work well for me (see my experiment
here 
)
If I was using slug bait, it would be something that they like, to tempt them away from my seedlings! Most slugs I believe are detrivores, so will go for stressed out plants (transplants) and dying leaves, so the discarded outer leaves of your lettuce will be preferred to healthy new starts.
By having a small healthy population of slugs I hope to feed a small healthy population of slug eating predators and employ them to keep the slug problem manageable for me.