Erik, it DOES matter. If you write your average good platform-independent code, it's all right.
But take this piece of code:

unsigned char high,low;
unsigned short word;

high=(unsigned char)(word / 256 );
low=(unsigned char)(word % 256 );

This will work on all platforms. But this one won't:

high=(unsigned char)(( word >> 8 ) & 0x00FF );
low=(unsigned char)( word & 0x00FF );

First one performs maths which are provided by system and just do the Right Thing. The second depends on byte order in a word.