Benzene is a planar molecule, so the dihedrals should be rigidly constrained around zero angle (or depending on your dihedral angle convention you are using angle might be pi). For instance, a harmonic potential with very large spring constant can be utilized to keep the dihedral around zero.

Thanks Hasan

But, my question includes a general case for any aromatic compound.

How to deal with C-C-C-C dihedrals in rings? Is it necessary or could be ignored?

Could you introduce me a references or FF?

You can look for the available FF e.g., Charmm, AMBER for aromatic amino acids.