My theory might be wrong, but I tend to go for the least hot powder.
As an example, in my Super Blackhawk I could use Bullseye, Unique or 2400.
I selected 2400 as the slowest burning.
Why? I want the barrel to last as long as possible.
From there I worked out the most accurate load for the cases, primers, and bullets selected.
FWIW I ended up with 19.5 gr. 2400, CCI magnum primers, and 250 gr. H&G Keith type mold.
Of course if your goals are different, say stretching the powder to the most rounds possible, etc. then Unique or Bullseye might've made sense.