Hi Steve, since Trim and booleans actually have curve projection or extrusion built in, you usually would use curve projection only for intermediate construction steps, like for instance see here for an example where curves are projected on to a sphere and then used as the paths for building sweeps:
http://moi3d.com/forum/index.php?webtag=MOI&msg=817.8

Working with individual surfaces where you would trim individual surfaces instead of solids tends to be a somewhat more advanced kind of toolset, it can sometimes be helpful to work at a surface level if you want to kind of disassemble and rework some particular area of an existing solid though. There is an object repair tutorial here that shows that kind of use in a lot more detail:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

But as far as building basic models goes, I would encourage you to focus more on using solids for doing basic shapes since for general stuff it tends to be more convenient and quick to use the solid toolset.

- Michael