If preview is enabled the server renders only a few frames of the job first.
Preview renderings have automatically a higher priority than the main render phase (+20).
Preview is very useful if you have many jobs.
After the previews are finished you...
...have an overview how long all yout jobs will take to render.
...can view all the pictures at the local website.
...have a quick overview if one of your render jobs is crap (e.g. wrong light is hided, wrong shader assigned, texture not found).
On the other side, if you disable preview rendering you need to make sure that...
...frame render times in high quality are reasonable.
(you get everything rendered until tomorrow/deadline/next scenes have to render)
...you have setup everything right in your scene.
(no helper objects still visible,
shader plugins installed on the farm,
all textures on the farm,
passes/layer visibility setup is right...).
The things you have to consider if you disable preview render do not sound very problematic and they usally do not happen. Exspecially if you have rendered the scene before.
But I personally know from my and others experiences that they do happen.
You add an object and forget a visibility or shader setting for a pass/layer. A whole night of render for nothing.
You are tweaking lights and shaders. Therefore you disable hair or particle simulations in a differnt part of the scene to work faster.
You tweak shaders in your scene and it does not take that much more time. But at render stage some models are replaced with highres-versions or your render settings (DOF, Mblur) are different, render times rise hours+.
You test the render times and they are good, but in the middle of the sequence is a small part where the camera moves close to an object. So it has to render a closeup, which can rise render times very high. Or even worse, this usally small object has a tiny frost reflection and your whole scene is visible in the reflection...
As you can see rendering the preview first has advantages. You can disable it (I also do it sometimes), but then the job should be a fast light-weight render job that you have already rendered some times.
Words at last:
Exspecially if the deadline is near and you are thinking of disabling the render preview (as you can gain a small percentage of render time), jobs that render wrong jeopardize the deadline. (And you usally make more of these small mistakes the more you have to hurry).