|You need a liner when the diaper is no longer holding all the pee--or you feel it's nearly to that point and are concerned that you might be getting to leakland soon, given a normal changing interval (nighttime or every few hours during the day) There's no real rule about using liners; some kids need them, some kids don't, but most do at some point need them at night. Some moms like to put in a bit more than is truly needed at night so the whole diaper doesn't get saturated, in order to have something dryish to hold when taking the diaper off. But of course that does add to size. I haven't used the XS Sandys, but wonder if using a liner with them would make them need a larger cover size to ensure complete coverage? Just speculating on that one, not that it would hurt to use a larger cover size, since you'll need it eventually anyway.