Compiling *is* a form of test, especially in languages which make extensive use of types such as [Haskell][1] or [ML][2]. In other languages it's a syntactic scan that tells you little.

Having said that, "compile as you go" seems to me a very situational habit. You could equally well be marked down for being "twitchy" for compiling more frequently than the interviewer's personal prejudice. It sounds like nitpicking; nobody likes to admit that an interviewee has *aced* the test; it tips the scales of the salary negotiation.

Not all build systems are fast. I worked on a (C++) project where [Make][3] would spend 30 seconds just [stat][4]'ting everything to determine whether it needed to build or not, and most files would take a couple of minutes to build if you had made changes. We were reluctant to do this more frequently than every 10-15 minutes. Someone will no doubt supply an anecdote of when compiling involved taking your deck of punch cards and carrying them to a different building ...

Compile when you feel you've done a complete conceptual unit in your head and are ready to have it validated. Once a minute or once a week depending on the workflow.

 [1]: http://en.wikipedia.org/wiki/Haskell_%28programming_language%29
 [2]: http://en.wikipedia.org/wiki/ML_(programming_language)
 [3]: http://en.wikipedia.org/wiki/Make_%28software%29
 [4]: http://en.wikipedia.org/wiki/Stat_%28system_call%29