Let me add another point on room temp. Lets assume one is growing organically in soil. The whole idea we have is to create a living soil with a diverse micro herd to basically care for our plants for us. So we take care of and feed the soil, in turn the soil takes care of and feeds our plants. The microbes won't function well and will die off with sustained cold temps. There's the plant, still alive in the pot in the cold room, but it ain't gonna work like we wanted it too.

I think as far as acceptable low temps, 55 with the lights off is the lowest you should ever allow, and yes allow, because the successful indoor grower must be able to completely control the growing environment.