Exactly, why would you expect temerature to change?
Sorry, I shouldn't make it sound like that's a bad question, you have every right to ask that, it's just something us old timers figure everyone else knows.
Again, it's a valid question and I thought Makerbot used to say in the porduct page descriptions the recommended temps, but with the new Replicator, they want everyone to use defaults anyway thus just checking the desciptions wasn't helpful.
In fact, both systems (3mm or 1.78) use the same nozzle output size so input size doesn't matter. The only thing is that you are smashing a 3mm fialment into a 0.4mm string and in theory, smashing a 1.75mm filament into a 0.4mm string is easier to feed. It them becomes like a gear ratio (in the settings) in that you feed more 1.75mm filament (length) to get the same extruded length of 0.4mm.
Only thing that matters is the material type for temperature