In general, mental ray, from inside XSI, only loads shaders from $XSI_BINDIR and $XSI_CPU_OPT.
Both these environment variables are set in setenv.bat (.xsi_7.X on Linux), unless the ray3rc location has been overridden by MI_ROOT.

So if you find that XSI is looking for shaders in the wrong place (eg you are getting black tiles/images rendered), then check that MI_ROOT is not set system-wide to an old location.

h/t: Halfdan