Optimal solution to develop a wordpress theme?

What i was doing previously, are as follows –

1. I copy the wordpress base theme (like twentytwelve in wordpress 3.5)
2. Rename the folder and converted index.html of my HTML to index.php for wordpress theme.
3. Replace style.css with my current style.css.

Is it the right way of theme development or some else better technique can be applied for theme development ?

I google a lot for the theme development but 99% tutorials are only using index.php , style.css only(some only header/footer etc) although in the new version of wordpress 3.5 twentytwelve I found there are too many files in the theme.

So for the client case I dont want to leave any core theme functionality provided these dummy themes, so is it the correct way I am following or for the best way I need to design and code for each file ?

I like to do this as follows:

1. Go to underscores.me, fill out the theme name, and download the theme.
2. Strip out the parts of this (somewhat clean) template that don’t want to use.
3. Start building your own template with the functionality that you wrote.

This way you will have a clean template to start with, and no core theme functionality provided by the theme you decided to use as a dummy.

I don’t know exactly if there is any “right” way to build your own theme, only that the Codex insist you use the WordPress Coding Standards, CSS Coding Standards and that you follow the design guidelines.
I think that, except of the parts of the Codex a developer is free in how to develop a theme.