Advice: Main Timeline or Object?

Hello guys:trout: ,

When I study the actionScript some tutorials, books, articles… put the action script on main timeline (_root) and somebody use onObject, what should I use or get to use to?
or some people prefer main timeline and some use scripting onObject? Is that it?

Please, give me advice or what you prefer and if you’ve got little time to explain it why?

:luigi: