Using primary and secondary forms are extremely important when modeling in both classes and in modeling for any object. when I model if it is not an organic object and it is made i ask myself how was this made and how many pieces was it made in to make this object and break it down. Edge quality is very important because there is no 90 degree angles and there is not straight lines in nature. It is important to have smoothing edges for the form to smooth correctly in 3 mode. It is also important to model you object to look like 3 mode in 1 mode for a better all around model. The only problem with holding edges is when you import it to Zbrush when you divide the mesh all the divisions go to the holding edges and you don't get the typology you need in the areas you want. Making normal maps are really fun to do because you can take a highly detailed model and map it onto a simplified version of itself and get the same detail.