Postgres at least will store rows roughly in insert order. So whether you're using integers or UUIDs for primary keys won't affect on-disk layout.
When scanning the index, the database will be traversing it in btree order, anyway, so as long as the values are comparable, ints or UUIDs shouldn't matter much.
When scanning the index, the database will be traversing it in btree order, anyway, so as long as the values are comparable, ints or UUIDs shouldn't matter much.