Lots of interesting comments on this thread, and some that go contrary to my experience.
If casting bullets from straight wheel weights, then you'll want to get the mold hot enough to drop frosty bullets. If the bullets are shiny, the mold isn't hot enough, and you will not get a complete fillout of the mold.
Heavily frosted bullets are no more brittle than shiny ones, once they have cooled. Yes, if you drop a barely solidified bullet from a mold onto a hard surface, the bullet will bend or even break in two. It's not brittle, it's just not solid!
As to water quenching, that is an entirely different subject, and yes it can be used to make a harder bullet, but it will be a brittle bullet at higher velocities. Some guns require the harder bullet to attain accuracy, but I haven't found this to always be the case. Also the brittle issue is related to impact velocity, a water quenched ww bullet at 1100-1200 fps is not going to be a brittle bullet.
460 gr .476" bullets launched at 1100 fps from a 480. Bullet on left through 38" of wet newsprint, bullet on right 2+" of bone and 10+" inches of wet newsprint. Bullets cast from ww dropped into a 5 gal bucket of water.
Wheel weights that are air cooled with a bhn of ~13 tend to be very predictable on game and don't suffer from brittleness with impact velocities up to 1800 fps. A properly fit bullet with a good lube isn't going to cause leading issues either.
Adding tin is an option to aid in fillout and to add some maleability. However it's an expensive way to go at ~#17/lb. 1-2% tin added to wheelweights will give you most of the advantages gained via tin without nearly doubling the cost of your bullets.