TOTALLY agree with this. But I heartily recommend a 3x3x4tall instead... because a 3x3x3 cannot fit two Rednet Ports, and having two of them installed in your reactor is critical IMO... put the two Rednet Ports atop one another on one face of your reactor, and connect them using 2 Rednet Cables. Then right click one of the Rednet Ports, select the icon with the Battery and +/-, and drag it into one of the colored slots (doesn't matter, I use the gray one) then click "Commit". Now exit out and go to the other Rednet Port, right click, select the icon for Control Rod Insertion Percentage, drag it into the same color slot you used for the Battery icon and click "Commit". Now your reactor is on auto-pilot and won't waste fuel. When you've got lots of stuff running in your base and the Reactor is called upon to output, say, 80% of it's capacity, the Rednet Ports will adjust the Control Rod Insertion to 20% (the inverse of the power output). Or, conversely, if you aren't doing anything and your Reactor is full of RF storage (0% output), the Rednet Ports will insert the Control Rods to a full 100%, effectively putting the Reactor into a "standby" mode where it uses almost zero fuel.
I'm tellin ya, the 3x3x4 is way way better than a 3x3x3 - not for the little bit extra power you get, but for those critical Rednet Ports to automate your reactor's power output and conserve your fuel. If you don't use Rednet Ports, you are guaranteed to leave your Reactor running full blast at some point, without any need for that kind of immediate power, and you are just wasting your Yellorium.