I know it depends on the species. The flats that are typically available often require colder temps, some like no higher than 65 degrees F. The African giants should be kept no lower than 76 degrees apparently (I just learned that. I thought they were good down to 72 but could do okay with nighttime drops to 68). I've been keeping my communal tanks between 70 and 78, with a typical of 76--purely due to what I keep the AC set at. Mine are doing okay, but I think need warmer temps to really thrive. I have one feather (a flat) and I need to see what temp it needs and if I need to move it out of the tanks I'm about to put heaters on.
As others have said, it varies a LOT between species! I keep my Harpaphe and Sigmoria millipedes at 65F and they've been okay with temporary drops to 54F. In contrast, Anadenobolus monilicornis need at least 72F.
Is there a particular species you're wondering about?