There is a longstanding debate in handcrafted soap making circles whether one should add a preservative to liquid soap to inhibit the growth of bacteria and fungi.
Most of the liquid soap makers I know of, including myself, do not add preservative to undiluted soap paste.
There is much less agreement about whether to add preservative to diluted liquid soap. Many soapers do not. Many do -- and I do. Of the ones who do not use preservative in liquid soap made for home use, however, some say they would add preservative if they sold the soap.
To further complicate this matter, very few of the many preservatives on the market are effective in the high pH of lye-based soap. I currently know of just three preservatives -- Suttocide A, Glydant Plus, and Liquid Germall Plus -- although using LGP in liquid soap is an off-label use. (1)
I'm not a microbiologist. I do not use preservative in my undiluted soap paste, but I do use preservative in diluted liquid soap. With those disclaimers, here's my take on whether or not to add a preservative to diluted liquid soap --
Have you ever had mold grow on jam? Even in the refrigerator and even with plenty of sugar in the jam for preservation, mold can and does grow, particularly around the edges of the jam where oxygen, light, and environmental conditions are more favorable for microbes to grow on exposed fruit particles.
Microbial growth on the surface of a soap bar is regularly washed off, so any microbial growth that does happen (and it does) is discouraged. A soap bar is also essentially solid, which is a physical barrier that discourages microbes from penetrating into the soap structure.
With a liquid soap in a container, any microbial contamination is not removed by use. The fluid nature of liquid soap also allows microbes to migrate around inside the container. Both of these aspects favor microbial growth rather than discourage it. Microbial growth often starts in the thin film of product at the edges of the container, on the lid, and on or in the dip tube and pump mechanism. It can grow within the main body of the product as well.
The dilution of soap with water to make a pourable product reduces the preservative effect of the soap itself. The more dilute the soap is, the less the soap can act as its own preservative. Diluted LS is very roughly 20% to 40% pure soap by weight. Bar soap (and liquid soap paste) is very roughly 70% to 90% pure soap by weight. That is a big difference.
By "pure soap," I mean the weights of the ingredients that actually turn into soap. That would be the fat plus the the NaOH or KOH. "Pure soap" does not include ingredients that do not turn into soap. This includes water, fragrance, color, and other additives.
One can use good sanitation when making and packaging liquid soap. And you can be smart about the kind of packaging used -- for example using a closed pump bottle rather than an open jar. Even so, there is no way you can entirely prevent microbes from being present in the packaged product. And there's no way a person can prevent the consumer from doing something that contaminates the product later on.
A broad spectrum preservative is insurance against the fungal and bacterial stragglers that sneak their way into the product and its container throughout the lifetime of the product.
What is the best way to know if your product is well preserved?
Know your preservative system is right for the product you're making, learn how to manufacture and package in a sanitary environment, do preliminary challenge testing on your own, then send the finished product to a challenge-testing lab.
What do I do personally?
I don't sell products that need preservation due to the greater responsibility for ensuring consumer safety. If I did, I would take my own advice. For my own personal use, I use a preservative in my diluted liquid soap (and lotion) per the manufacturer's recommendations, work in a reasonably sanitary way, make small amounts of preserved products so I can use them up in a relatively short time, and watch them like a hawk for any changes in appearance, texture, and odor.
1. Barber, Jane. Preservatives: All you need to know! http://www.makingskincare.com/preservatives/