Testing non-central features
Only 1% of our clients use particular features or scenarios. Do you think we should spend as much time testing these features and scenarios as we spend on our central features and scenarios, or we should test mostly the "happy path" for these features?
As already stated in a comment, it depends.
You have to consider a lot of things in order to decide your test strategy. Usually you have limited resources for testing (time, budget, testers, infrastructure etc.), so you cannot test everything equally thorough. A risk-based approach helps you decide where to concentrate your testing efforts and how thorough to test.
For this, you should first identify all relevant risks, and assess them. The assessment includes categorizing each risk and determining the likelihood of occurrence and impact upon occurrency for each risk. The impact should be assessed by all stakeholders, that is, you should involve your customer(s)/clients.
This helps you deciding which test techniques to use and the priorities of each test case or test object of your system. The effort associated with developing and executing a test is proportional to the extent of the risk.
For the particular features you asked about, some questions, which you should answer yourself, are:
- Would it be a showstopper for your clients if these features would not work as expected (business risk)?
- How important are those clients to you? If you lose a client, this could have a very negative impact to your image (business risk).
- Is the implementation of those features very complex or has many interfaces to other parts of the system? Or is it quite simple (technical risk)? If it is complex, you should test it more thorough than if it's simple.
Of course, there are many other questions you have to answer for yourself before you can decide your approach.