You cannot rely on all variables to be initialised by startup code.

If you try to remember which ones are, and which are not, initialised by the startup code - sooner or later you will get it wrong.

Hence the safest rule is to insist that all variables must be explicitly iniialised (or assigned a vlaue) before use.

void my_function( void )
{
   u8 uninitilised;
   u8 initialised = 0;
   u8 assigned;

   :

   assigned = something;
   // now we can safely do stuff that reads 'assigned'...
   
}