Building an own theme is only necessary if the client has very specific needs. Customizing an existing theme saves a lot of time, but must have the functionalities we need. Think about:

• available regions for the blocks
• fluid or fixed
• how easy we can modify it. If the client has an existing site, probably they like to adopt some elements in the new site, like the header. I prefer a theme based on the PHPTemplate engine. This an easy way of tweaking the theme. The Fusion theme and sub themes are really easy to customize. 

I choose a theme that complies the specifications and think fits the customer. Colorschemes can be changed easily later on. I stick to one theme. Because I first make a proposal that serves only as a starting point, I don't waste much time on it.

I make a new customized theme of my own so that when the original is updated, it won't write over any changes. It's as easy as copy/pasting the original theme  folder into the same location (sites>all>themes), changing the folder name to "my-theme" and editing the .info file with my-theme name. I don't forget to change the project name in the .info file also or it will still register as the original theme and Drupal will get angry if you don't update it to the newest version.