To me, this question is one of your dilligence, and where you're willing to put in your time/effort.
For me, I don't keep an up-to-date portfolio while I'm in a position and not looking for something else (thus why my portfolio site is pretty barren at the moment.) Simply put, keeping your portfolio up to date is hard. If you don't have the time, don't feel guilty about it. Just make sure that part is locked and loaded before you head out on your next job search.
There are some great benefits to keeping your portfolio up-to-date, if you have the time:
- SEO - web crawlers are more likely to gain better keyword recognition and rank your site higher the more relevant content you include.
- Ease-of-use- you won't have to spend a large chunk of time re-designing or adding content when you want to move on to something else.
- People will find you - having a great portfolio site means opportunities are more likely to come to you, rather than the other way around.
TL:DR: Update if you can, but don't feel guilty if you can't.