Sounds like a very interesting question! Could different plant distributions in tanks have different temperature optima for best growth? It's not hard to imagine that a few degrees one way or the other might make a big difference in an enclosed, highly driven system like a brightly-lit CO2 aquarium. Mostly I've just seen folks say any temp in the range x->y will work OK. Is this really known to be the case for plant growth (not fish health)? Have people played around with temperature change when they're having problems of one sort or another and found that it's just not important?