I've found that if I do nothing at all, I end up at higher risk for chaffing and other uncomfortableness. Depending on conditions, I've also found that small scratches will get infected much easier if I'm not practicing any hygiene at all, and so even a quick wipe-down is a good way to make sure I've not missed anything.
As for temps, I've found that a quick cool off before going to bed can mean that your metabolism is headed upward, so while you are not moving, it ends up warmer, where as a quick warm-up if over-done leads to metabolism dropping, thus leaving a person cooler. It really depends on the person though, so everyone is going to be somewhat different. But I do think that is why a lot of people do find that they find a temp change. Even if all you do is change clothes, that's a lot of water you are not evaporating (and thus cooling). The salt content won't matter much at the temps for body cooling, as the marginal higher "boiling" point actually means more heat leaving per molecule of water, but it's a very small percentage for a human.
Soap for me is any sort of liquid Castile style soap (dr bronners is the famous one, but there are many out there) and so long as it goes into the soil away from a water supply, it will decompose fine.
I've also found that pre-trip habits matter a lot. Deodorants and such can leave people much more rank than a few days of clean sweating (so long as they are washing) Body chemistry plays into this as well, so foods eaten and all that does matter. Suffice to say, everyone really does find their own pattern with practice.