I have the best results sizing to the chamber throat diameter, most of which are a little larger than the groove size of the gun used. Softer cast bullets will slug up to the throat diameter better than hard ones and can be sized closer to the groove dia, but harder cast bullets might need to be a little larger (without going nuts here) as they won't slug up and fill the throat.
I had a real problem with some really hard .430 bullets in a .429 Super Blackhawk leading badly at lower (1000 fps) velocities. Throats measured .434-.435. Sizing to .433 (largest I could find) solved the problem as did pushing the .430 ones out a little faster.