Why is unnecessary to include the mass of an electron when determining the mass of an atom?

Mass of electron is so very less that it is negligible

Well, to be perfectly precise you would have to include the e- 
but since it takes more than 1700 e- to match the mass of just one proton 
adding the mass of e- takes the result out to several places beyond significance. However, the measured atomic mass of any atom DOES include e-, of course, because they are there in the atom as it is being measured.