If we purchase a seat for an infant (and install a car seat in the seat), is it still permissible to end up doing infant-in-arms? (i.e., if the baby is fussy and wants to be in our arms). Or if you have a car seat installed in a seat, does that mean they don't let you do infant-in-arms?

Edit: I'm thinking mostly of take-off/landing/when the seat-belt sign is on. Obviously, during other flight conditions, we could always have the baby in our arms.