Answer by Joel Lewenstein:
It's important to have the ability to be a perfectionist, and the discipline to only use this ability when necessary.
Much design work, especially visual design or typography, will be working through many details: Looking at color relationships, whitespace, line spacing, page composition and balance, etc. until all the visual relationships work together to form the polished final product. To work through each of these details effectively, you'll need to develop the ability to recognize and name the details that need work, experience to know what types of solutions will work, and the perseverance and patience to keep trying alternatives until you find the right solution. This perfectionism (which I would rephrase as "attention to, and caring about, the detail work") is necessary for producing a high quality, polished final product.
As important as this ability, though, is knowing when and why to use it. At the risk of oversimplification, I'd say it's worth perfecting a design if lots of people will be using it frequently for a long period of time. This means that any final product (a new physical device, a website's homepage, a key flow in a mobile app) should be be perfected before shipping, to ensure that the experience of interacting with the product is as refined and pleasurable as it can be. But there are also many examples where that perfectionism is a waste of the valuable resource of time: If you're working on an early or experimental prototype, it's better to focus on learning about the bigger concepts than getting each visual decision right; if you're building an experiment that validates a concept but wouldn't launch without further refinement, you can avoid doing the final level of polish; similarly, a minor settings page that is infrequently used may not be worth finishing all the polish. This mostly applies to a small, resource-constrained teams where the opportunity cost of perfectionism (an inherently time consuming process) is high.