Less compiles into valid CSS, but note that "valid" does not always mean you will have compiled code of the highest quality. Lint your final CSS code with CSS Lint; this will help you to find possible issues and prevent you from making common mistakes. It not only checks your code for possible errors, such as double or empty properties, but also checks for compatibility and looks for problematic patterns or signs of inefficiency.
CSS Lint applies a set of rules to your code; all rules are optional and you can even write your own rules. It also has rules to check the maintainability and accessibility of your code. Finally, CSS Lint applies some rules that are based on the principles of Object Oriented CSS (OOCSS). More information about OOCSS can be found at http://www.smashingmagazine.com/2011/12/12/an-introduction-to-object-oriented-css-oocss/.