It depends on how you ride and how you use 'em.
If you are going to sit on the saddle all the time and just need barends to switch handpositions, you may want to have them pointing slightly upward.
If you are a sportive rider and want to use your barends to get off the saddle and pull on the handlebar while climbing hills (like me), you may want them pointing more forward.
I have mine at about 30 degrees i guess and find them perfect for having less strain on my wrists while climbing.