Do you start a Timer if you need no global timer just for the purpose to have this event triggered random number generator?
No, I use whatever timer I have running for whatever purpose. The beauty is that for random, you can read the timer on the fly, there is no need to stop it for reading. If I did not have a timer running constantly (I can not recall that ever being the case) and had a free timer available, I'd just use that

Sometimes you have no Timer running all the time
well, if you do not, and do not have a spare, then, obviously you can not use this method.

Erik