commerce_product_calculate_sell_price() doesn't always return the same result and is not safe to be used on its own?

If you use commerce_product_calculate_sell_price() inside hook_preprocess_node in your template.php similar to this:

  /**
   * Implements hook_preprocess_node().
   */
  function MYMODULE_preprocess_node(&$variables) {
    $node = $variables['node'];
    $node_wrapper = entity_metadata_wrapper('node', $node);
    $products = $node_wrapper->field_product->value();
    $price = commerce_product_calculate_sell_price($products[0]);
  }

Then the price array returned will have all price components doubled up because the product you supplied to commerce_product_calculate_sell_price() had its price already calculated. Meaning you get tax applied twice, discounts applied twice etc.

But if you do something like this:

  $product = commerce_product_load(1);
  $price = commerce_product_calculate_sell_price($products);

Then the price will be correct because the product you supplied didn't have the price calculated beforehand.

I was very surprised to discover this. I've always taken it for granted that the commerce_product_calculate_sell_price() returned an absolute price, but looking in the product_pricing module it seems to me you have to render the price field itself such that commerce_product_pricing_commerce_price_field_formatter_prepare_view() is invoked to be sure that the price is actually reset before the new components are applied. This explains why the "original" part of the price array is not always there for me either.

I very rarely render fields as I feel the overhead of cleaning up all the bloat in the output is too great. I prefer fetching things from the database and outputting them myself, using functions like commerce_product_calculate_sell_price() where I have to.

Am I doing something wrong here? Is this by design? Do you have to render price fields to ensure you're not getting the wrong price?