I'm surpised Dan hasn't jumped on this one.
The Young's Modulus, which is a measure of a material's stiffness, for Ti is about half that of steel. So "Yes, Ti is not as stiff as steel."
However, you also have to ask "What is stiff enough?"
I can't answer that, but I do know several people who have built rifles on Remington Ti actions.
They have very accurate guns and are very happy with them.
Lucho