For ph, anywhere in the 6.0-8.0 is fine for most fish we buy. It's best to have a stable ph then one that fluctuates which is what usually happens when you try to alter it. Just let it be.
For temps, 78-80°F is good for bettas. Yes, this one will matter. If the water is too cool for them, they'll start losing their color, not eating, acting lethargic, get sick, and eventually die.