I use sodium lactate in every batch of cold process soap I make. However, the other day I was making a soap (getting ready for Christmas gifts) and I got interrupted. I forgot to add the sodium lactate and let's just say it was an issue getting that out of the tall and slender soap mold.
The loaf was much softer and stuck to the sides of the mold rather tenaciously. I put it into the fridge for about 6 hours and then it finally released and came out okay with a few dents or misshapen areas on the soap loaf. I even spray the insides of my molds with a food grade silicone spray to help with the release of the soap loaf and still with no sodium lactate the loaf was sticking.
The batches I use with sodium lactate always... I mean always, come out of the mold firmer and very little sticking to the mold. I use it in the ratio of 2% to total oils weight. I add it to the cooled lye water mixing very well to ensure it's dispersed well, just prior to adding the lye solution to the oils.
I believe that adding it to the cooled lye solution is the key. Some have stated to add it to the oils, tried that once and didn't work. My mind said to me: "Hey self... since SL is in an water solution @ 60% concentration, doesn't it make more sense to mix it into the lye/water solution?" I did some more research and discovered it really does it's cold process magic when it's dissolved in the liquid rather than the oils.
Yes and
@Johnez is correct in stating that adding SL at the end of the hot process does make more fluid and easier to spoon or ladle into a mold. I use it when making my shave soaps. It makes the soap go from a pasty consistency to a heavy custard-like state. Of course, elbow grease, a large silicone spatula and a decent tater masher help to get the SL integrated into the cooked soap.
You do what works best for you though! We can all get to a similar result without having to take the exact same route.
What I described above works best for me personally. I am not implying I am totally correct in the manner I do things related to my CP soap and the loaf's firmness and sticking to the molds, I just know that this process works for me, so therefore I stay with this method.
As long as we get results we're looking for that's all that matters.