I've mastered quite a few Materia and they have been "born". Now, is it worth taking these new ones and getting them again mastered to sell them or just dump them. I have only sold one Materia and that is Mastered All for a butt load of cash. I have also found myself not levelling up any materia for example..Steal, Elemental, Exit, MP Absorb, HP Absorb, Added Effect, Deathblow. I find myself thinking I am doing myself a diservice by not. But as far as I can see the only reason to level up those I have mentioned is to just spawn them again. If that's the case I won't feel as bad.

try to keep a mastered materia of each kind. If some remain unmastered, then try to master them too. You can win a "master materia" that does everything later in the game, with them.

^^Yea, techno gave some good advice, you can get a Master Materia of each kind (magic, command, etc.) that allows each variation of that materia to be available. Pretty awesome imo.

And if you sold a Mastered All for 1.4MILLION gil, I don't think you need much more gil do you?

Just keep selling mastered materia until the game overloads with gil and crashes.

Your best bet is selling mastered All Materia. You get a new one so the added effect isn't really lost. Specially late game when you really only need one spell to dispatch a group of foes. They sel for a nice 1.4 mil Gil.