How aggressive do you need to be in Poker? Do you need to be aggressive to win?
You will need to change your playing style many times within a tournament. And it is not a constantly increasing spiral of aggression.
When should you be aggressive? I dont think you can narrow it down a great deal (in terms of when exactly to change, and what to), apart from the fact that aggression levels should generally increase in the lead up to the money, and as tables become shorthanded (eg final table).
Each tournament is different. And adaptability is important.
At the beginning you should obviously consider blind speeds, starting chips and payment structure. Is TAG really what you want for this particular tourney?, and if so what form of TAG? The structure of some games may dictate a more rapid development of your play, whereas some tourneys may be a leisurely stroll of postflop poker. Prepare by analysing the structure a little.
During the tournament keep an eye on your M level (chipstack divided by cost of one rotation). If this starts to get low (say less than 8) then you are going to have to get aggressive and fast. Do not allow yourself to get blinded away gradually. Even it it means launching with nothing. It is better to launch with position with 10 Big Blinds, and 52offsuit, than it is to launch with A7 and only 2 Big blinds. Your opponents simply wont have any equity in folding to you in the latter scenario.
Dont get overly concerned too early about who has what stacksize in the tournament, worry more about whether you personally have the chips to see some flops and play some proper poker. You dont want to be under any pressure to get all your chips in the middle, and you want some room to make a few plays. If you have an M of 30 then you know you have enough chips to play some poker.
As the play evolves your strategy can and should change alot according to how your stack develops or dimishes, relative stack size, blind level, how your table image is currently at the table, how the others at your table are playing, and how specifically a player you are up against is playing. If a player cant evolve their play continuosly in order to take into account all these factors, then they are doomed in the longterm.