Just an example how you could strip spaces before all (well, almost) control structures. See: https://github.com/TiGR/twig-preprocessor

Example.php

<?php

$loader = new Twig_Loader_Preprocessor(new Twig_Loader_Filesystem('tpl'),
    function ($template) {
        static $regExp;
        // the RE isn't perfect, it won't match structures having curly braces within, 
        // but it's okay for me.
        if (!isset($regExp)) {
            $regExp = str_replace(
                ['_',      '{',  '}'],
                ['[\t ]*', '\{', '\}'],
                '/^_({([#%])[^}]*[^-](?2)}|{%_block_\w*_%}{%_endblock_%})$/m'
            );
        }
        return preg_replace($regExp, '$1', $template);
    });
$twig = new Twig_Environment($loader, array('cache' => 'cache/tpl'));

Preprocessor.php

<?php

/**
 * Twig Preprocessor loader that allows adding custom text filters for template
 * strings.
 * 
 * @package Twig
 * @author Igor Tarasov <tarasov.igor@gmail.com>
 */
class Twig_Loader_Preprocessor implements Twig_LoaderInterface, Twig_ExistsLoaderInterface {
    private $realLoader, $callback;
    
    /**
     * Constructor
     * 
     * @param Twig_LoaderInterface $loader loader that does real loading of templates
     * @param callable $callback processing callback that should accept template 
     *                           string as the only parameter and return the result
     */
    public function __construct(Twig_LoaderInterface $loader, callable $callback)
    {
        $this->realLoader = $loader;
        $this->callback = $callback;
    }

    /**
     * {@inheritdoc}
     */
    public function getSource($name)
    {
        return call_user_func($this->callback, $this->realLoader->getSource($name));
    }

    /**
     * {@inheritdoc}
     */
    public function exists($name)
    {
        if ($this->realLoader instanceof Twig_ExistsLoaderInterface) {
            return $this->realLoader->exists($name);
        }
        
        return true;
    }

    /**
     * {@inheritdoc}
     */
    public function getCacheKey($name)
    {
        return $this->realLoader->getCacheKey($name);
    }

    /**
     * {@inheritdoc}
     */
    public function isFresh($name, $time)
    {
        return $this->realLoader->isFresh($name, $time);
    }
}

?>

Version for both twig versions 1 and 2: https://github.com/TiGR/twig-preprocessor