You don’t want to be totally neutral but very slightly positive. Once you start your breath hold, you burn O2 and produce CO2. CO2 is denser so at the end of your dynamic you’ll be LESS buoyant. So if you start a little positive, you will be about neutral at the end.
I'm pretty new to freediving, and I don't mean to be rude, but this doesn't make any sense whatsoever. It is true that at the same pressure, volume for volume, CO2 weighs more than O2, but:
a) A human body holding it's breath doesn't magically gain mass. The extra mass would have to come from somewhere. In this case the extra carbon atoms are a waste product of your metabolic processes, so they are only in your lungs because they aren't somewhere else in your body.
b) Say you have a lung capacity of 6 litres and 21% of that is O2 at the start of your dynamic. Even if you replaced allthe O2 in your lungs with CO2 (which I suspect is incompatible with being conscious/alive), you'd only have 1.26 litres of CO2 at the end of your dynamic. The difference in mass between 1.26L of O2 and 1.26L of CO2 is utterly negligible in this context.
If you're losing bouyancy during a dynamic it will be for some other reason.