Are you asking about regular training, or racing? And if racing, what distance are you asking about.

If you are asking about training --- training involves a bunch of different types of running. For Example, tempo runs, long runs, hills, intervals, just running to run, etc Each of those types of runs calls for a different effort, mind set, and goals.

If I am running a long run I am not going out fast, and will not even try to finish fast. If I am running intervals I am working on speed. The goal is to gain speed. To run faster one has to run faster, but not every day.

I hope this helps

I agree with skypilot in that it depends on many different factors (what you are preparing for, your expectations, the course, the distance and so on).  I tend to start out faster for shorter races.  I know this is an overly simplistic answer, but, in general, it really comes down to individual sustaining options for a particular length of time. 

I have never been a strong finisher, so usually I find it more beneficial to begin with greater speed and plan that the final few miles will probably be at a reduced pace. 

If you are talking about race events, this sometimes works out fine to place yourself in the upper top 25% early on, but it still depends on all the other variables. 

As a general practice, I try to start out fast with a prayer that my "afterburner" will kick in and I will just go into that numb mental haze of continuing to move quickly. 

Again, everyone is different, so it is important to indentify your personal advantages. 

There is no right answer to this questions.  From a physiological standpoint, it is common to give in to the adrenaline rush at the start and go out fast.  Unfortunately, that rush wont last long and you end up going out so fast that you doom the rest of your race/run.  If you really concentrate on remaining in control at the beginning and getting into a good rhythm, you may be better able to run faster as the race progresses.  If you get really lucky, youll get a nice endorphin high that will carry you.  I can tell you that I have experienced that many times but not once when I went out too fast.  Every person is different though!  This will require you to continue experimenting as to which works best for you!  Good luck!