In my experience and in my conditions, species like ventricosa (Philippines), Sanguinea (Malaysia), as well as veitchii (Borneo) and alata from high altitudes (Philippines) stop pitchering when night minima are above about 19C. Day temperature maxima don't seem to have a huge amount of influence, they seem to pitcher with very hot days provided the nights get cool enough. This is consistent over the last 7 years I've been growing these species under their current conditions.
All this, and all other evidence around, is anecdotal. Someone would need to do extensive control studies to see what the results are like. I'd love to know what other factors change the equation - does higher humidity make highland species pitcher with higher night minima, or does it make no difference? Do lowland species cope with lower temps is humidity is lower? This sort of factor could be one reason why results vary from grower to grower.
As I said before, the view that Philippino species are more tolerant of different temperature ranges than species from other locales arose probably from comments made by Joel on his Nepenthes Around the House site, and also by the fact that the most ubiquitous species and one of the easiest to grow species - ventricosa (and you could probably include truncata) - is from the Philippines. Is this empirical? Well, it would basically have to be shown that most Philippines species follow this trend, and species from other regions don't. I admit that ventricosa are truncata are hardy, tolerant, and easy to grow. But so are sanguinea, maxima, tentaculata, bongso and others not found in the Philippines (and I've found the latter 3 more tolerant of warm nights than sanguinea and ventricosa; veitchii and alata appear to vary depending on source altitude). Conversely, merrilliana, bellii, argentii and to a certain extent sibuyanensis can be quite temperamental and unforgiving.
Other interesting points to look at would be the exact altitude various species are found (and unfortunately some of this data is conflicting and suspect), and whether species with a broad altitudinal range are more tolerant of, for example temperature fluctuations, than those from a more limited altitudinal range. Distance from the equator might also be considered.
What have other growers found? I want someone to post who has completely contradictory experiences!