Well, to tell you the truth, some printers may prefer that you simply place long vertical and horizontal lines behind the Art layers, that peek out behind a white filled box at the edges of the page.
This way they can extend the lengths of the lines to reach
their paper size which may be HUGE, especially when they're printing 'several up.'
Therefore Filter > Create > Trim Marks, would be a quick way for you to generate some properly positioned lines quickly, since they are 'editable' afterwards, unlike 'Crop marks.'
Whether you extend/connect the trim marks is probably a personal pref., but giving the printer something editable is a good idea.
Sef.
It is alright to decorate construction, but never construct decoration. - Pugin, on Arch.