When to add vegetables to stew?
I have played around with this quite a bit myself. For me, boiling and then adding does not work. As suggested in derobert's remark, many of the veg traditionally added to a stew are there not just as filling but to give off their flavor to the stewing liquid. Leaving them out during the stew is detrimental to your flavor. 

I have gotten my best results by adding the stock vegetables (carrots, unions, celery, garlic) as early as I normally would and then adding some vegetables late in the process to provide a differing texture to the whole. After some experimentation I have found that quickly stirfrying some green veg gives a very nice effect, as it adds some color and a very different texture to your end result. My personal favorite for this is quartered sprouts, quickly fried at high heat and then dunked in icewater.

Purely from a texture point of view you vcan also get nice results with diced root vegetables such as cabbage turnip and turnip-rooted celery treated in a similar way.