Indeed, "_ne_" can always be omitted in _spoken_ French ("_ne ... que_", "_ne ... jamais_", ...).

If you are trying to speak or write in a formal way however, I would recommend _never_ to omit it In a document, never omit them (just as I would advise never to write "don't" in a formal document but always "do not").

When spoken, it is also usual to shorten it, just pronouncing the _N_ (a sort of a contraction). When speaking formally, "_Je n'suis pas d'accord._" will be better than "_Je suis pas d'accord._"