Risk make ti bolts for just about everything - make sure you get the Ti washers also!
I reckon Ti bolts are 50% lighter on average so it should be possible to save 75-100g if you did every bolt. I also like them as they stay new looking for ages, I hate the fact that we can spend £5K on a new bike and it comes with cheap nasty pig iron bolts!
If on a tight budget though I would swap the disc rotor bolts for Ti because that will reduce rotational resistance, albeit only by a small amount - but it will help.