Category, Header, Single Templates. “If is” Query vs. Pages

I am still trying to grasp making my site not too intensive on the server. I wanted to know the pros and cons to building a site with many templates and which way to go. What would be your thoughts on:

1. I have one header, one category, one single, and one sidebar, and add If (is)) functions to determine the template and scripts that pop up on the page.

2. I have a few header pages, few singles, few category pages etc. and add less If (is)) functions.

3. Have a header, category, sidebar, and single page for every template.

I wanted to know the pros and cons, since at this time I am using about 12-14 if(is)) statements to make my template (3 in header, 3 in single, 3 in cat, and 4-6 in sidebar. Would it be better to make custom pages for these templates, or are the if statements ok. Any criticism/advice is welcome.