you can do either.
tucking them in means extra leak protection and easier to put the cover on covering them all.
its abit trial and error - sometimes tucking them in its tricky not to pinch bubs skin when doing it (i am abit unco lol) and it can leave marks on their skin.
i also found sometimes leaving them out left marks on skin
the marks never bothered bub and faded very quick.
have fun!
eta - i have only ever tucked in with a baby beehind bamboo (BBB) on a small bub)