I heated my garage with standard propane heaters while fiberglassing my boat over the winter. I was using polyester resin, and the propane heat posed no problems. MEKP, the catalyst, is very flammable, but once it's mixed in the resin, there is no problem*. You just have to be careful when handling the MEKP by itself (never near a flame; I briefly went outside when I needed to pour MEKP).
Not sure what you heard about propane heat and curing of resin; I didn't come across that when I was researching it and as I said, I had no problems with curing, with polyester resin, anyway.
* This may not be true in a really high volume fiberglassing operating, like running a chopper gun, but for standard layups like we do when DIYing, I would not worry about it.