easiest way to cut down bolts. Use a vice grip to softly clamp the head of the bolt so you don't need to hold it.
Thread a nut of same metric size past the spot you want to cut. Use a permanent marker to set where you want to cut. Use a dremel with a cut off wheel. Put on safety glasses. Set dremel to 20,000 rpm. Then cut the spot where your marker indicated. Use the flat part of the cut off wheel to taper the end a bit and remove burrs.
Let bolt cool down before touching it. Remove vice grips. Use an allen key to hold the head of the bolt as you unthread the nut off. The nut will clean up the thread that tiny bit to form the correct thread pitch at the very start of the bolt.
as to where to use Ti:
Brake caliper mounting bolts
Lever clamp bolts
Disc rotor mounting bolts
seatpost binder bolt
cable pinch bolts (I prefer Ti over Aluminum as the aluminum hex wears over time as you keep on changing cables)
suspension mounting points (for Mountain bikes, but most mountain bikes now come with speciality fasteners already fully optimized)
Places to use aluminum bolts:
Water cage bolts
Under bottom bracket cable cable mounting bolt
rim brake nut (the one in the fork and frame), make sure it's full length and not hanging on by a thread
You can use aluminum bolts for the cable pinch bolts
headset preload bolt
computer mounting bolts (I.e. garmin mounts etc)
A good source for Ti and Al bolts:http://www.torontocycles.com
they offer bolts with cut threads instead of rolled with a die. But I have never had a problem.