Search Engine Optimisation
I have considerable experience with web site advertising, my main business is IT, my wife is running a carpet cleaning business, hence why I am on this site. Anyway, what you describe is called keyword density.
Its very important. There are 2 aspects of advertising, pay per clikc (like google adword) and free advertiisng if the search enginge recognises your site. Each search engine provider uses a "secret" algorithium for searching your site. This takes into account many factors, one being keyword density and relevance. So yes getting this right is important, per web page you have. BUT if you over do it, the algorithium will show that you are trying to spam it! and you will go down the rankings.
For your website, you should have within the 1st 1k of text your keyword, with reasonable keyword density say 30%. The remainder of pager should have lower desnity of keyword typically 2%.
Relevant content on your site is very improtant too. Aim for at least 20 pages, if not more.
Links to external sites, and more importantly links to your site, are recognised as key factors with seearch engines (doorway pages and keyword/site rings are important, google these terms).
I will stop rattling on now, as I could go on and on...