The rule of thumb is to be sure that you have practiced using your entire art creation tool chain. You don't want to have to learn how to export a mesh after the Jam begins. Expect format woes or exporter bugs the first time round. One way to avoid these headaches during the Jam itself is to test your content creation pipeline beforehand.

Create a sample sprite or mesh, a tiny game level, and an example sound effect. Export them all in the proper formats. Embed or import this data into your game engine. Compile a simple "hello world" example game and ensure that your art appears perfectly. Test whether your animations run, your sounds are audible, and that the empty game prototype runs without errors.

If you can do all of the above, your game creation tools are ready for Jamming. A wise warrior knows that sharpening your sword before battle is smarter than trying to do it on your enemy's armor.