: Given the years of built up grunge would you still skip the soap?
: At one house it was stored in the garage but under the front
: porch so occasionally big storms would blow rain onto the porch
: and then down onto the boat so I'm really not sure what is on
: it.

I'd give it a good wash/scrub with detergent and water before I started sanding (and hit it with solvent if there is anything that the soapy water won't budge).
You don't want to 'sand in' the dirt, and even a few grains of sand or grit can cause a lot of deep scratches.
I think the idea is to make sure that the hull is rinsed with clean water and wiped dry before proceeding to more epoxy or varnishing.