Question

Why can we ignore the disposition of the lone pairs on terminal atoms?

Answer 1

Basically that depends on the shape of the molecule, but in general terms we can just ignore them because they are terminal atoms, and those lone pairs are not contribuing in anything to the molecule. They are just there to do electron repulsion which causes (among other reasons) the shape of the molecule and it's respective hybridation.

Lone pairs are usually electrons pair that are not appealed to anything else, for example for the following molecule:

The molecule has 14 valence electrons. There are four bonding electron pairs around carbon. The shape is tetrahedral. The bond angles will not be exactly 109.5° because the terminal atoms are not all identical. The molecule CH3F is an AX4 species. The lone pairs on the fluorine atom are not considered because it is a terminal atom.

Hope this helps