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In that case, you should be reviewing whatever case study you feature on a periodic basis to make sure it's reflective of your current skill level.

Depending on how much time you sink into learning, you can progress at a very high rate. You want people to make the most informed decision possible when hiring you, so you want to be showing them the thing most likely to wow them.

Keep in mind that some companies with an IT Department may still outsource smaller projects to freelancers. In this case, your development practices may be evaluated by IT minded folks. You really do not want to lose out at a job because you are showing code 4 years old, and that's not even how you'd go about it if you were to start over.

Another poster did say something about traffic increase. But let's extend that. If you can prove that you not only redid the design, but you increased performance by X%, and traffic by Y% and so on, those are real numbers people who make decisions can base you on. So it may be worth your while to learn about metrics.

In School, I would really look into a course on statistics. You'd be surprised how often that comes in handy.