Most glandes you see are blunt, but from a video I watched, some people think they became blunt through the years of deterioration, and originally were sharp. So... which?

depends on how sophisticated the casting was.
often there would be ledge of lead and general all roung rough edges.

Given that glandes drill their way through things, then pointy ends and rough casting would actually increase the damge done, while not effecting flight performance all that much.

Rough edges would also increase flight noise. So would be psychologically advantageous.
Rough edges would also give better purchase on a sling pouch and impart greater spin to the missile.

So a rough and ready casting would actually be better in many ways than a really clean smooth one.

It's reasonable to guess that people used various methods based on their own experience, preferences, traditions, and applications. Particularly when there are advantages to either blunt and smooth vs. sharp and rough.

I wonder how much skill would be required to throw bullets so that they consistently stick pointy ended in? 

what the picture doesn't show is the fact that they were slightly flattened, which helps drill.

The whole point of a glande shape is that as long as it spins along the central/long axis it will self orient in flight to travel point first.  Imparting spin is important.

It's to do with either centripetal or centrifugal force of some kind - depending on which pedant you ask

This is the video I'm talking about. https://m.youtube.com/watch?v=Tpu-BCSfJ2c

I gave measures on this site once, at least for the Greek ones I knew and had handled. Basically a it longer than 3cm, a bit less than 1.5 cm high, and say 1.2cm thick-- i.e the section was ellipsoid rather than flat. There are spanish lead projectiles that are quite flat.

Maybe there was also different types of ammo depending on the mission/engagement.

A skilled slinger would be familiar with the characteristics of different projectiles. For a long range bombardment I would prefer a biconical with distance characteristics. We all know what those are if you've ever slung lead. If you trained every day, you would also know how far a 1oz, 2oz and 3oz projectile would go.

If my slinging was going to be 100m or less, I might want a flatter, sharper or perhaps longer projectile that would buzz, rip out chunks of skin and scare people.

Perhaps a well stocked armoury contained a few different variations to choose from.

I'm guessing this variation has more to do with the mission, lead being so easy to cast I'm sure you could probably tell them how many/what type of bullets you would need.

I'd love to go back in time and see how much we've gotten wrong when it comes to slinging.

Most sling bullets weigh around 30-40g; but there are a good number of very heavy slugs which clock in at 100+g. They're immediately noticeable in museum collections. If these were indeed for slings, and not e.g. catapults, then I supposed they were special ammo (distance or impact ?)