You should complete the DLC as soon as you can after Priority:Mars. 

IMO he is a very valuable squad member with very unique dialog options. I was able to do it fairly early in the game.

Adding Javik to your squad does net you some unique dialogue options when interacting with characters. (Not posting it here. But checkout [Javik/Unique Dialog](http://masseffect.wikia.com/wiki/Javik/Unique_dialogue) here.)

If Javik and Liara are on the mission Priority:Thessia, then that conversation adds a lot of lore, since at that point you are at a Prothean place where they posted a record of their attempt to fight the Reapers, as the Asari call it a temple and Javik is a Prothean. 

One last note, you can not take him on Priority:Palaven.