That depends on the context. A neutron has approximately the same mass as a proton, and approximately 1800 times the mass of an electron. Whether that is negligible depends on the problem at hand.
The mass of a proton is about 1.672621777 x 10-27 kg. The mass of a neutron is about 1.674927351 x 10-27 kg. The equivalent energy of a proton is 938.272046 MeV/c2. The equivalent energy of a neutron...