What's the best way to setup a conditional to see if a matrix block is used? I need to add some javascript to a page depending upon whether or not a matrix block is used, one is for a slideshow block to load responsiveslides.js, another is for a video block to load fitvid.js. If I place it in the loop then I will get the javascript repeated for each time an entry uses that matrix block. I'm looking to add this towards the bottom of the template and load the .js files once each if the block is used in a loop above. What's the best way to do this?
I would use a loop to set a conditional, and then check it afterwards. You need to initialize the conditional outside of the loop, otherwise it won't be set outside the loop later.

{% set haveAVideo = false %}
{% for block in matrixBlocks %}
  {% if block.type == 'video' %}
    {% set haveAVideo = true %}
  {% endif %}
{% endfor %}

{% if haveAVideo %}
  {# video block js #}
  {% includeJsFile ... %}
{% endif %}