They will comfortably live a couple of degrees below and above that. If you are comfortable and they seem to be thriving, you've a system (or ecology if you wish) of feeding and water changing which is working. Sit back (for all three minutes before you are needed elsewhere) and enjoy the guppies.

There are other livebearers which really need the high 70s, low 80s - the Limia species from the big islands in the Gulf of Mexico & the Carribean for instance - but my modest experience and a little reading would suggest that the only group of the commercial livebearers that pretty much must be kept 78-80 F. are the mollies. And that is strange, because several wild strains and ancestral species of the commercial strains sometimes live in conditions where temperatures go lower than that. On occasion, they are also more sensitive to crowding than the other commercial livebearers. Another set of those "go figure" situations.