as far as i know the trial versions do not have a file that notices when you have installed xlC or vac.C, etc.. I suspect it has to do with support and when you are selling programs compiled with the compilers. I guess you should talk to your legal department about the risk(s) you take by not licensing.

If you are concerned about the validity of a license - I recommend either getting a license - or downloading and using the gcc compiler that is part of the AIX Toolkit. Sometimes a port requires/is easier when compiling with gcc. Some developers think gcc is the only compiler - or standard - and use new features because they are there, rather than "limiting" themselves to ANSI C standards.

One important reason to use the IBM compiler is to be able to get hardware (i.e. POWER architecture) related optimizations for CPU intensive applications. vac/xlC and gcc are equally portable - as long as gcc specific syntax is not used. Gcc has no licensing fee.