Is it just a requirement that you reg a business name before you buy your IE domain name?

Any reason why you could not buy the domain and set the business up at a later date...eg to secure the name before word gets out.

Why would I want to regster a business name other than others might grab it?
What is the advantage to registering a business name?

Would I be liable for tax if I setup a website and business name ahead of getting the business of the ground?

I would like to test the water before I jump in is that such a bad thing?

Thanks,

Kajo.