If you talk with people who must use their Taekwondo regularly, such as bouncers, bounty hunters, police officers, or just rowdy drunks, they will consistently tell you the person who hits first usually wins.
So we practice reaction, but experience says that action beats reaction. Therefore, you could say that we are training to lose or at least give the advantage to our opponent.
There is a variety of types of assault that you may be subjected to and statistically some happen more often than others, the habitual acts of violence. Therefore, it should be possible to rank all assaults in order of the most likely to occur to the least likely to occur. For example, which is more likely to happen in a street scenario; someone attacks with a right hand swinging punch or with a side thrust kick. Of course, the punch is much more likely. The top probability attacks are the ones to which we should focus our training since they are the ones we will likely have to defend against.
The most probable attacks are techniques such as grabs, pushes, pointing, gesturing, etc. Many of these attacks escalate from simple verbal attacks to physical assault. Once a physical attack is eminent, a preemptive strike is may be used most effectively, however, it may lead to legal problems later.
Depending on the situation, a preemptive strike may be necessary, even if there may be legal or civil consequences afterward. The decision is yours.