Bestway to write php functions

I have seen that every theme use this format

if( !function_exists( 'blogrock__f' )) :
   function blogrock__f () {
   /// code here
  }
endif;
add_action('after_setup_theme', 'blogrock_setup');

I am pretty bored writing if.. endif blocks. I know I can create new classes to avoid this . But can I do this in this way?

if (!function_exists('theme_add_next_page_button')) { 
function theme_add_next_page_button(){
   // function blocks
   }
add_filter( 'mce_buttons', 'theme_add_next_page_button', 1, 2 );    
}

Can I remove function conflict by this way? If there are otherways,what are those? And does my later way impact negative on theme performance?

Thanks