Using a metal spudger might damage the bezel and/or screen. It's definitely doable to separate the two without a metal spudger - I did it myself with no problems - but other iPhone display assemblies may be tougher than the one I took apart.
Sometimes using the metal spudger is necessary-- my screen was entirely too shattered for the suction cup to hold. I slowly pried it open, and yes, there was slight damage to the plastic.
