It has to come down to a judgement call at the end of the day but I sometimes ask when I'm not sure. This will usually be when I've got a party composition that I'm not that confident in for big pulls, say NIN, BRD and AST in low end gear for the dungeon. On the other hand if I have BLM, SMN and WHM and everyone is well geared then I'll pull big and not ask, if a party like that can't handle big pulls at this stage of the game then... well they need to learn somehow. On the rare occasion I have 2 melee then I'll pull one pack at a time as hitting positionals and not having to constantly dodge seems to speed things up more than pulling big and relying on AoE from jobs that don't excel at it.

I'm always happy to adjust though based on how the first pull goes. The new expert roulette dungeons seem to have been designed to allow groups to do this imo, the first pulls are all pretty tame and heavily gated so I see how they go and make a judgement after them.