Generally, you are more prone to surrendering stiffs in higher counts. As far as I know you always surrender 16 v T, no matter the count. Most surrenders have a negative index where hitting is better. Vs ace is sort of a special case in that actually at higher counts the ace will bust more and this is late surrender so the dealer has already checked for blackjack (whereas vs ten the dealer COULD have a 10 down card for a total of 20 and in higher counts this becomes more likely) . But as far as I know you just keep surrendering 16 v A in higher counts as well.