No, yes, maybe It depends...
It needed to be added only to events subroutine. Eg click, keypress, keydown, timer tick, etc... For MCS I think it is just few events for editor box(timer, keypress, keydown, keyup).
If you call any subroutine or function, and error happens in function it will point to that function. So I can usually narrow down to single function. Then I usually menage to recreate that event, or if I can I'll add that error handler to that function, then make new exe, and wait for someone to report error.
I usually put error handler while writing code.
For some non critical stuff(changing txtbox color, formatting, preventing user to enter something wrong, etc) I just put "on error resume next". So it will just go over error, and probably with next char error should be gone, and formatted correctly.