In the Common Pitfalls & Anti-Patterns (CP&AP) it is stated that

It is not recommended to rebuild your indexes unless you absolutely need to and if you need to do this often then it is advised to determine why and to try to resolve the underlying problem.

In version7.5.x (or even earlier?) there's no separate "Optimize index" button anymore. We still need to optimize the index and if this non-optimized index happens a lot, does it mean there is an underlying problem? Or is it normal to have a non-optimized index? Meaning that Umbraco will pick it up at a later time and optimize what needs to be optimized.