There’s no particular guideline.
The SmartLighting rules you create through the smartlighting smartapp are the only parts of the system at present that can “run locally“, that is that will still operate even if the Internet is out or the smartthings cloud is unavailable. So a lot of people always start with those because they want to get the local operation.
The design intention pretty clearly is to have everyone use automations unless they are power users who will eventually be writing custom code through the “rules API.“ However, as of this writing, December 2020, there are still somethings you just can’t do with the built-in automation creator. So they go to Smartapps to fill in the gaps.
So as far as what most people do, I think most people who just buy the system and set it up without ever coming to this forum just use automations.
People who do more research and want local operations put as much as they can into the smartlighting smartapp.
People who do a lot of research and are comfortable with custom code use custom code when there are features that they want that are not supported by the built-in automation creator. Sometimes that’s because they bought a device/service which has advanced features that require a smartapp, sometimes it’s because they asked a question in the forum and the answer was custom code.
Other than smart lighting and the third party RBoy smartapps library, the most popular smartapp for Power users is unquestionably webcore, which is a very advanced rules engine that can do a lot more than the built-in rules engine can. But that’s a whole other conversation. 
See the community FAQ (the topic title is a clickable link)
And the following is a very good example of a feature that should be in the built-in automations but just isn’t, so people have turned to smartapps to fill the gap: