Drawing symmetrical objects in perspective can be a challenge. In situations like this, I generally use Google Sketchup to help me with my accuracy This video explores the powerful tools that Sketchup provides for working with repeated shapes and symmetrical forms. Even if you're saying "Yeah, yeah - symmetry is no big deal in perspective..." would you change your tune if I asked you to draw an object that 7-sided with rotational symmetry? Sometimes a little bit of 3D saves a lot of time.
The goal of making thumbnail sketches is speed. You're trying to get a lot of ideas onto the page as quickly as possible. This video explores the concept of symmetrical thumbnail designs. Specifically, it offers a single-button solution to create these horizontally mirrored drawings. Photoshop actions can be a huge time-saver if you know how to use them - and this 'mirror action' is no exception.