<p>Some guys did experiments on the pre-padding and post-padding in their paper <a href="https://arxiv.org/pdf/1903.07288.pdf" rel="nofollow noreferrer">Effects of Padding on LSTMs and CNNs</a>. Here is their conclusion.</p>
<blockquote>
<p>For LSTMs, the accuracy of post-padding (50.117%) is <strong>way</strong> less than pre-padding (80.321%).</p>
<p>Pre-padding and post padding doesn’t matter much to CNN because unlike LSTMs, CNNs don’t try to remember stuff from the previous output, but instead tries to find pattern in the given data.</p>
</blockquote>
<p>I have never expected such big effect of padding positions, so I suggest you verify it yourself.</p>
Right padding vs left padding word vector?
<p>In RNN world, does it matter which end of the word vector is padded so they all have the same length?</p>

<p>Example</p>

<pre><code>pad_left = [0, 0, 0, 0, 5, 4, 3, 2] 
pad_right = [5, 4, 3, 2, 0, 0, 0, 0]
</code></pre>