To add to this - in a previous post I asked for advice regarding keeping sharp edges in parts of a model after subdividing - my model is complex, so cutting in edge re-enforcement is not an option, it would become too confusing and messy. After a lot of experimentation, it turns out that changing the default edge smooth setting to rounded, with a value of 3, for a single subdivision gives a perfectly sharp edge - use 2 for a softer edge - without all the extra lines. Seems that the value must be 1 higher than the number of subdivisions for a soft edge and 2 higher for a sharp, hard edge.
Just thought I'd share this, for what its worth