Too Aggressive with 99 Heads-Up?

[DougI]

I play a lot of one table tournaments and do a good job at becoming one of the final two spots.
Now, I am usually very aggressive with PP and want to know whether my strategy is flawed or not.

There are situations that happen time and time again HU where I tend to put the other player all-in and I always notice that I am basically playing for a coin flip.
Let me explain.

After the tourney winds down to 2 players, I now am pretty confident on how the other guy plays.
We are about even in chips. He raises for about a quarter of my chips. At this point, I believe he is playing me with two overcards or Ax suited. If I go all-in with a pair of 99 to JJ, I feel it will be a horse race.
Sometimes I win, sometimes I lose.
Is it better in these situations, to see the flop and if no Ace appears then go all-in, or is my aggressive all-in move the best course of action every time on decent PP heads up?