We've all heard the advice: don't add salt when cooking lentils, because they'll toughen or not become soft or variations on that theme. For example, on lentils.org:  Be sure to season with salt after cooking – if salt is added before, the lentils will become tough.  The thing is, the same advice is given for basically all dried legumes, but Serious Eats debunked that - at least for beans (specifically,  cannellini beans). But lentils aren't much like beans, neither in size, cooked texture, nor even really in cooking processes.1 So I'm not sure if the Serious Eats experiment really applies to lentils. Has anyone either confirmed or debunked the don't-add-salt thing specifically for lentils? 1For example, soaking lentils in my experience just adds to the preparation time: it doesn't seem to make an appreciable difference in how long it takes to cook them, but it adds the time you had to take to soak them. 
If you look at lentil recipes, there is not agreement on whether to salt or not.  In fact, this one begins with a salt brine...others say salt makes beans tough.  I think the salt making beans (lentils included) tough is a myth has been debunked. This post refers to Harold McGee, who I would trust on the issue. It is acid that keeps beans from softening. The real issue with salt is that evaporation during cooking will increase your salt concentration.  If you don't keep track of this, you can end up with overly salty beans.  That, in and of itself, might be a good reason to wait, almost until completion, to salt your beans. 