if we are providing a seperate "printer friendly page" for each page of site then do we need to use print media css in site?
only screen css is enough in this case?
You don't need to use print CSS, but you should.
It's the smart way to do "printer friendly" pages, anyway -- you don't need to do any extra coding.
Many users never see or use "Print" or "Printer-friendly" buttons either. It's much more natural just to hit Control-P.
I've never had a user complain that the ads, banners, and menus disappeared when they printed a page. ;-)
ETA:
Some helpful tips for print CSS are found at this Stack Overflow question.