Hey, if you were to make a puddle of soder/metal.. i would suggest just squishing a sphere in polys.. then adding a lattice:

HOTBOX > Deform > Lattice

Then pulling the edges and whatnot to your desired shape and pulling the bottom of the sphere to be flat on the ground, like gravity has pulled it into that position.

For rule of thumb.. I now will start out my organic models with nurbs, then convert to polygons because they are so flexible. I have not done much work in Subd's and i dont intend to for some time. If you are doing a non-organic model (robot, building etc.) then I would suggest using all polygons. Thats just me though, the more you model the more you kinda just "Know" what you want to use. Like babyduck said.. everyone is different, to an extent.