<p>If you're making a number of pieces that should be identical, you're best off making a batch of them at one time. Take pasitive steps, gang sanding or cutting for example, to make them as close to identical as possible. Check each against the others. Making two brake ducts which are mirror images and curved in 3 dimensions, starting with 4 identical tops-or-bottoms and 4 identical left-or-right sides is good. Much easier than cutting out four pieces separately, making one duct working each piece against the drawing, then cutting out another four and building the second duct.
In a perfect world, it makes no difference. ..
Its even better to start with the joint dimensions spot on  what you want, so you're done when you glue them, but the length a bit over so you can trim the pair to match and be correct when the pieces are all together.
After snapping out the four top and bottom pieces, I sanded them down to correct dimensions. One of the tricks to the world at large is when you can cut things to net size, and just smooth the cut faces, and when the cutting process is going to leave a lot of roughness and irregularity that will have to be separately cut away or abraded away. With styrene, you'll find cutting to near-net size and then sanding/filing to finish size works well. Its soft stuff and when snapping, sawing, or especially clipping or whittling, the work winds up more irregular than you've have at first expected.
See the variations in width in the pieces I'm holding?
</p>
 IMG_0608
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/9998127@N06">Bill Abbott</a>
CC BY-SA 2.0
<p><a rel="nofollow" class="external text" href="https://www.flickr.com/photos/wbaiv/3275566773/">Matched parts for air ducts</a>
</p>
<ul><li>Uploaded by <a href="//commons.wikimedia.org/wiki/User:FAEP" title="User:FAEP">FAEP</a></li></ul>