Normally if you've paid for the upgrade on WordPress to remove the footer on the site and the advertisement this is a good start. However, on a free theme, you'll still need to customize the theme and make it your own. This isn't an issue with people who work on blog sites and understand how to customize the these. Most people won't pay for a premium these or not. Most people who sign up with WordPress will select a free theme over a paid one any day of the week. This just depends on how you modify the theme to suit your needs. When people visit a website or even a blog they are not worried if you paid for your theme or not. They visit for the content that is on the site and how the site is put together. I personally don't think this matters one way or another. The only thing that matter is that you have paid for your name and your name is registered. If you keep the wordpress.com extension this is when people don't have much trust in your website at all.