Around 3 - 5 is fairly normal for a solid TAG game. Having said that, poker isnt really a game where you aim for certain stats. If you start blindly firing at pots to bring your aggression up, or blindly calling when you would normally raise to bring it down it will only hurt your game. Stats which are different to normal stats are an indicator that there may be a problem with your game. But there is more than one way to beat this game, and the optimum will vary depending on your specific table/opponents. If your aggression is noticably low then think about spots where it would perhaps be more profitable to be more aggressive, but dont just go betting at everything because it will fix your numbers.

The same goes for any stat. It is far more important to understand the concepts behind why you should make each individual play than it is to fit your numbers to a standard profile.