I've tried may different temps. Room temp no heater 65-70. With heater 74-75. With heater 76-77. I've personally settled on 74ish. This way there is no Winter and my shrimps pretty much breeds year round. The breeding still picks up in the warmer months though when the room temp will exceed the set 74 and go up to 76-78.
I figure even if it shortens their life by a little if I get two more clutches of babies out of one female then I'm ahead of the game.