My advice for newbies is to follow the advice that is put here as I do not think newbies have the experience to spot early signs of disease, or to spot early signs of distress in fish etc etc so you should increase your chances by keeping the fish to the warmer end of the scale.
However, with a bit of experience, you start to realise that you can push the boundaries a little and we all know that one of the problems with fishkeeping is that the tanks breed almost as fast as the fish. I have well over 100 now and I would not be able to have that many if I needed a heater for each one.
Some fish are able to withstand cooler temperatures and I believe that this is actually good for the fish because it creates a period where the fish do not breed and they seem to rest and recouperate and come back far stronger when the temperatures rise.
That said it is important to know what your fish can withstand. Through experience I have learned which fish can and which fish cannot withstand low temperatures, but if your experience is different, stick with your own advice. But it is important to know that there is another way, even if you choose to ignore it.
In some wild species e.g. the Skiffias, too high a temperatures may cause infertility and even in cultivated stock it can lead to short life spans.
One thing to remember is we are talking about temperate species, not cold water species. Xiphophorus evelynae has been reported to overwinter outside, but I do not believe it and there is a new type of hybrid Gambusia which is also supposed to be the same, but I wouldn't try it myself.
Even in the wild, there are some pretty cold habitats in the warmer countries. In Mexico for instance, although the country is quite hot, there are fish which come from higher altitude in the mountains and these fish get cold conditions; whilst in other cooler areas, there are fish which live in warm spring fed rivers which need warmer, stable conditions all the time.