Statement I

Formal charge on the two terminal nitrogen atoms in ${\mathrm{N}}_3^{-}$(azide ion) is same in all different possible lewis structures

Statement II

Formal charge on an atom is independent of lewis structure

Formal charge on an atom in a given species may are may not be same in different possible Lewis structures 

$\mathrm{N}=\mathrm{N}=\mathrm{N}$ formal charge on terminal nitrogen atoms is the same,  -1.

$\mathrm{N}\equiv \mathrm{N}-\mathrm{N}$, formal charge on one terminal nitrogen atom is 0 and other is -2.