You should use an edging - the bark/gravel will migrate all over the place otherwise. You could use a lot of materials for edgings, but timber or precast concrete are the most common and are cheap.
The treated timber ones will degrade over 10 or 15 years, but you may prefer how they look.
Precast edging will last forever, although it should be haunched in a mortar bed, otherwise they'll settle and end up looking gash.