I render all my designs (shoes) in Illustrator. I put most shading and highlighting (for presentation purposes) on their own respective layers. I hide those layers when preparing the rendering as a spec sheet, and put all specs on a separate layer as well. I export them to JPEG and email to factory. I’ve found that 150 ppi JPEGs suffice for our sample rooms.
After first round of samples I usually make revisions in one of two ways: 1) Using the original Illustrator format, I highlight all revised specs in yellow, or 2) hand-write my revisions, using Photoshop and Wacom pen, on photos of the first-round sample. The method I use is determined by the extent and nature of the revisions and/or simply the mood I’m in.