Hi Cos,
I always suggest to set Lead Sources. It is also good to have a Initial Lead Source which is just set the first time and a "Last Touch Lead Source" which is always set new when he submits several forms. I like it to have as much information as possible, for example maybe one Whitepaper is the last touch before a opportunity is closed in most cases. That information would be very good.
You can set this when submitting forms. If you want to you could also do it using a Smart Campaign for visiting a very important landing page without a form.
Best regards
Steffen