Every caster precasts.
Summoner precasts Ruin III
BLM precast Blizzard III

The reason why they do that is to be optimal.
There is literally 0 reason why RDM should not hardcast a Veraero/Thunder 5 seconds before the pull. It's stronger than Jolt II, gives you a direct chance to proc if you use it without accel, it grants you dualcast. This is very simple to understand, really.
Since it's prepull, the cast time doesn't even matter. The fight hasn't started yet. At that point it's a simple comparison, 230 potency and 6 mana and impactful proc OR 270 potency, 11 mana and fire/stone proc. The choice is quite obvious.
You can stay whereever you want with RDM, staying in melee ranged just helps for little bits since you won't have to run into the boss to use certain attacks, assuming corps a corps is down because it should be used for DPS. I prefer to stay in melee range because corps a corps most of the time is on CD so I don't rely on it to get in range.