Indenting preprocessor directives reduces the code readability, because it make preprocessor directives harder to spot.

Noncompliant Code Example
void optimal()
{
  #if INTEL             /* Noncompliant - hard to spot */
  specificIntelStuff();
  #endif                /* Noncompliant - hard to spot */
}
Compliant Solution
void optimal()
{
#if INTEL               /* Compliant */
  specificIntelStuff();
#endif                  /* Compliant */
}