It is a common practice to download the latest Magento stable edition, install it, and tweak the default demo template until it fits your desired look & feel. The problem with this is that the default magento template was not designed for this purpose. The default Magento theme was designed to showcase all the bells and whistles that Magento has to offer right out of the box.
The main reason for this thinking is SEO. While the default Magento theme definitely has a lot to offer, it will not provide any advantage with regard to SEO over the competition. Especially now that it is very likely that your competitor has a Magento based shopping cart as well and t is very likely they are using a modified version of the default theme. If you start with the Magento blank theme or the Magento SEO theme offered by Yoast you are probably much more likely to achieve higher ranking for similar catalogs than if you started tweaking from the default theme. This is mainly because the default theme has a lot more fluff than one really needs on every page, especially on the product pages.
Another reason why starting with a blank theme is preferred is Performance. By now, it is known that Magento is a relatively heavy shopping cart. What I always advise my customers is to rethink your shopping cart experience from your users experience. Do you really need to show how many items is in your cart on every page? or are your users buy one item at a time anyway? is it helpful to show 100+ products on the front page or does it make sense to help your users get directed to the right subsection of categories and help them find the product they really wanted to begin with?
You can probably see where I am going with this. Think your shopping cart through, always refer to successful websites like apple.com and remember that in today’s world less is more. In essence, if you start with the blank magento theme, you are more likely to avoid including unnecessary features on your shopping cart that hinder on the site’s performance.
Did you find any difference on your site when you started from the blank template?