"Right action, right time" Eric Ries

The key is to invest in the parts of the product that need that attention. In the early parts of a startup (MVP) then it's all about building something to put in front of users.

As that scales, then you can spend some time paying down the technical debt by getting good Unit Test coverage, and eventually building automated deployments and an integration server.

I would spend more time trying to get to some level of Product/Market fit first before spending too much time on development processes that might not matter if you fail to build anything someone wants.

You'll usually feel the impact of what's missing as users report more bugs, or the # of defects your dealing with weekly outweight the # of features.

Right time, right action. Start simple.

(If you want something more concrete, just do 80/20 - 80% on user facing / value creating stories, 20% of paying down code debt / automation).