For feeders it is probably okay as they won't have a very long life anyway. Keeping fish in less than ideal conditions does tend to shorten their lives. So if these are fancy guppies you want to keep for awhile I would really recommend a heater or at least a room with a stable temp of at least the mid 70's.
Well I had a heater malfuntion were it spiked then dropped, 3 died but then
I kept one female successfully wihtout a heater for about a month but once I got a new one she seemed much happier and active
I've kept fancy guppies in a cold water tank for many months after I moved. They all lived and seemed to do just fine. What I found out though is that they dont breed as rapidly as they do in a heated tank. They will appreicate a heated tank over a cold water tank anyday.
bettagurl I have a feeder guppy that has lived for 3 years and is still with me and I have 2 other feeders that have lived long and are still here. I personally perfer them over the more fancy ones, although I do have some fancies. As long as your careful about what feeders you get they will be in good shape for a long time.
I have kept fanices in an unheated tank for over a year (70-72*F). I intended to use them as feeders but they are the fancy type, and they have multiplied much quicker than I use them. I pretty much stopped using them as feeders, now I am thiking about trading them all in at a lfs for store credit.