Can you guess which bolt is made from titanium in this photo? The bolt on the right is Ti. When is it right to use a Ti Bolt? I have developed a formula over the years. W(weight in grams) x L (lenght in mm) = X. When I was a kid I had no Idea what titanium was, I just wanted to go fast. My first bits of titanium were for my cherished Klein. A 118 bar from Bontrager, and some Titanium crank bolts.
Actually, there is no formula... and in most cases an aluminum bolt is lighter and less expensive. I will use titanium for bolts over 20mm and when soft aluminum threads are not strong enough. I needed a solution to allow me to use my Reynolds Ouzo Pro seat post. I went from a M5x30 Stainless bolt to a M5X35 Ti bolt to solve my thick carbon-fiber seat rail issue. The titanium saved me 5 grams at a cost of $15. You can make your own formula to figure out if titanium is worth the cost.