Is it important to assign CSS injector and JS injector code snippets to specific pages instead of site wide? Is it important to assign CSS injector and JS injector code snippets to specific pages instead of site wide?

If the code is a few lines, is there a REAL performance benefit to assign css and JS to only some pages? 

Or does it only matter for very large code?

I'm just wondering if it's over kill to target the specific pages all the time?

An example code snippet combo I'm using is the first word for JS

$('p').each(function() {
var word = $(this).html();
var index = word.indexOf(' ');
if(index == -1) {
    index = word.length;
}
$(this).html('&lt;span class="first-word"&gt;' + word.substring(0, index) +    '&lt;/span&gt;' + word.substring(index, word.length));
});


And CSS:

.first-word { font-style:italic; }


So less than 10 lines of code...
For those amount of lines, it's no biggie, specially if you're caching, as those lines gets cached as HTML.

An alternative, the good practice, would be to add the css to your theme's style.css file and the JS to your theme.info file. That way you can aggregate and compress them along with your other files.

Aggregating and compressing makes a huge difference. You should do that if you aren't already. (speaking of all your css and js files).