Most exotics have garanteed stat roll (ex: Impossible Machine can roll Int/Str or Dis/Str, but not Int/Dis) that are related to their unique traits (ex: The Ram has a garanteed Str roll). Most exotics also have one or two garanteed perk in their customisation tree (ex: Impossible Machine all have Snap Discharge). You should check out which stats and perks of any exotic gear are available for Glass Needle rerolling, check out if the version sold at the Exotic Blueprint screen already has the setup you were hoping for then see if you really need to replace the currently owned setup before spending these.