My own rule of thumb is that every theme that I create is responsive. Thus, if you want me to theme for you, you will get a responsive theme as a result. Sometimes designers design their themes in Photoshop or Fireworks for wide screens or very wide screens, without really thinking what will happen for the mobile and narrow screens. If I see design patterns that won't scale down well, I will warn you about it. In some cases reworking or simplification will solve the issue, or agreeing on how the elements will stack.
For theming, I prefer a starter theme with built-in grid. By default, I use Twitter Bootstrap as a starter theme, which takes care of many a design pattern and grid layout of the box. However, I can use other grid systems and starter themes, like Omega, Zen, AdaptiveTheme, Zurb Foundation and Skeleton Theme if client so requests.
In my work, I use HTML5 and CSS3, CSS with the SCSS preprocessor for maintainability. You can request theming to be done in LESS or pure CSS without a preprocessor if there is a specific need for that.