I wonder if that's all there is to consider though...
for example, this info from one of the testing laboratories says that many cleaners have either a low enough or a high enough PH that they don't require preservatives, as the environment doesn't support microbial growth:
http://www.antimicrobialtestlaboratories.com/Information_About_Preservatives.htm
I have seen others state that liquid soap doesn't need it while cream soap does, so I'm still wondering what other differences would account for that, aside from water content. It is interesting to ponder, and I'd like to find an answer at some point (me and my curiosity! lol)
In the meantime, I err on the safe side.