One reason that the TAG style is advocated for beginners is that it helps keep you out of marginal situations post-flop. However, if you want to progress to the next level, it is critical to gain those post-flop skills because you can't wait for TAG range hands and hope that at the same time somebody else has enough of a hand to give you his whole stack, and hope that this happens often enough to keep you surviving.The major weakness with playing only TAG style is that if you get nothing but one crap hand after another, you're handcuffed and the clock is always ticking in tournaments.
You can't always start out tight, in some situations I will be playing LAG right out of the gate because I'm at a passive table with people playing fit-or-fold and want to take control. This is a delicious situation to be in, especially when people keep calling your raises then folding to c-bets on the flop, and you definitely do not want to miss out on this.
The flip side is when you're at a table where every pot is heavily contested with a lot of 3-betting, check-raising, etc, then you should tighten up unless you are really confident of your ability to outplay your opponents postflop, and that takes a lot of experience and study.
If there was one single winning strategy, I would say that it's whatever strategy that's based on the types of players you're up against that forces your opponents to make the most mistakes, and this is going to be different for each and every game.