I don't think it has to be 5 minutes; so long as everyone uses the same time frame. If that where to be 10 minutes or 7 minutes and 27.34 seconds it wouldn't matter to me. The point is, you can cut the log where you want to inflate the numbers.......that should not be happening (see my example above regarding golf for reason as to why it matters on a list such as this).
I guess it depends on the goal of the thread.
I feel like trimming the parse to where you like gives less variability as people will post their highest dps.
If you are asking for ball park average numbers that a spec will pull over x amount of time then I would agree with you.
However that still doesn't prevent people from timing their biggest hits to land right at that time mark and inflating their dps.
If I understand your goal then I don't think setting a fixed time period will completely solve the problem although it would deflate dps numbers and be a better representation of what a class can pull.
I guess it depends on what you want to see.