This is why I come here, lol. I'm guessing it's more of a "feel" thing, but how long do you typically wait? I was just waiting until the sprue looked dull, and then cutting it.
It depends on the mold / bullet size you are casting. Larger bullets take longer to solidify. I have a Master Caster and it is very easy to get going too quickly. I usually wait until the sprue solidifies, wait another 3 seconds and then dump the mold. That is for my 9mm and .38 molds. The .44's I leave another second or two longer. You will have to determine how long to wait by experimenting a little.
Your bullets are certainly useable as is. There is the potential of the void left in the base being off center causing a bit of wobble. I wouldn't hesitate to use them as I am not such a gifted marksman that I would be able to tell!
Auggie D.