Personally I don't think much of such [-]intellectual shortcuts[/-] rules of thumb. As with almost everything else in life, there are many factors to consider and "it depends".
There are many tiny companies that I would be loath to invest 5% of my portfolio in. On the other hand, there are some large companies, internally diversified across multiple industries and geographies, that I would be quite comfortable investing 10% in (e.g., GE or Berkshire Hathaway).
I would argue the opposite: if you work at company X, you are already exposed to its future success / failure, and have no need to compound that exposure by investing in its stock. An exception could certainly be made for those high enough in management to have inside information about the companies' prospects (although such executives are typically subject to trading restrictions).