Many players hesitate in taking an aggressive shot because they are afraid of missing. On the contrary, if you are afraid of taking a specific shot, then (unless it's a championship match), any good coach will tell you that's a reason to take that shot, so you'll learn to do it under pressure - and especially when you need it when you do have a championship match.
The more you go for the right aggressive shot under pressure the better you'll get at it. In fact, an experienced player will be nervous about not taking the right shot, since he knows he's not playing the percentages, and so is literally playing to lose. This doesn't mean killing every ball; it means learning what the high-percentage shot is, and taking that shot whenever it comes up, regardless of the score. A great example of this is looping or otherwise attacking a deep serve. If you are afraid to do this in practice, then how will you do it when you really need the shot?