Do I need serializable for this query?

I have a table that keeps running totals.  It is possible that this
would get called twice simultaneously for the same UserID.  Do I need to
put this in a serializable transaction?  Since this gets called often,
could it be a performance problem if I make it serializable?

CREATE FUNCTION UpdateTotals(IN UserID int,
                              IN AddToCount int,
                              IN AddToSize bigint)
RETURNS void AS $$
BEGIN
    UPDATE
        Totals
    SET
        TotalSize = TotalSize + $2,
        TotalCount = TotalCount + $3
    WHERE
        UserID = $1;
    END IF;
END
$$ LANGUAGE 'plpgsql' VOLATILE;