I also read(search for IERS) that leap seconds are only added on June 30th or December 31st, so bi-annual updates should suffice.
But you don't want to update your lists at the times leap seconds are added (or skipped). We're talking about Date::Calc, and some people use it to calculate dates in the future. You want to update your lists as soon as it has been
decided a leap second will be added or skipped. I don't know how often people meet to determine whether we'll need another leap second, but I doubt it's more often than four times a year.