YES it does matter.
If they are close to the same temp, fine. If lye is hot and oils are room temperature, also fine.
If oils are too hot/significantly hotter than lye - VOLCANO!!!
One day, I decided to premix a couple 2-lb batches of oils and premix a couple batches of lye water to go with them because it turns out the measuring is my least favorite part of the soaping. When I soaped one of those premixed batches, the oils and the lye water were both at room temp so I knew, without use of the thermometers, they were the same temperature. I've never temped anything since.
But those digital thermometers make great swirling sticks!
I have found that my coconut once melted and the other oils/butters added doesn't solidify. However if it's a bit cold in the house it does get cloudy so I just give it a quick 30-60 seconds in the microwave then add my lye solution.
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