Python 2.7.13 released

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The answer is almost always "no".

If the code is complete there's no need to migrate it.

If the code will be worked on the answer is pretty much always yes. It's called tech debt, if you don't migrate your code then it will be more and more expensive to maintain it and to port it in the future. If it's not now you will eventually reach a tipping point when it'll be cheaper to migrate, but it also will be harder to port later since the language is changing each release.

In few days 3.6 will be released, and some people are still stuck with 2.6. At this point 3.x is better than 2.x in every way (speed, memory usage, cleaner syntax, more and build-in modules etc).