Okay, so if you're mixing evening and morning, I would cool the evening milk. This is not so much for the bacteria as it is for the fat. The lipase in milk will act on the fat and raise the level of the fatty acids. It often leads to off-flavors or an excessively goaty tang.
I cool evening milk, but not the morning milk, and combine them.
If you are doing all raw milk, completely without cultures, then you need to ripen the evening and morning mix until the pH drops by .1 or so. You need to make sure there's active culture. Most often, when not using culture, a whey starter from the previous batch, or a clabber is used.
I usually use a pinch of culture to ensure it acidifies, just in case, or use a stable clabber. It's somewhat tough to use the milk as is, there's less consistency among batches.