If you're planning on STS, or any higher class (STX, SP (street prepared), SM, etc) most aftermarket chassis and suspension braces are legal. However, the amount of gain in performance that you will receive is negligible, especially compared to the benefits of improving the driver, tires, springs, shocks, swaybars, and alignment.

This is because taking your typical street car to a higher level depends mostly on increasing traction (tires) and roll stiffness (springs, swaybars, shocks). If you don't have enough traction to even keep your car on your intended line, there isn't going to be much energy going into the body to flex the suspension mounting points.

Front to rear balance (ie eliminating prominent understeer) can be achieved with springs, shocks, swaybars, and alignment. At this point, you will have enough cornering ability to be concerned about slight changes in camber or toe based on chassis flex/deflection of suspension mounting points. But I could argue that you should upgrade all of your suspension bushings to urethane before you should worry about how much your chassis is flexing.

What level driver are you? Small nuances in handling won't be readily apparent until you've had a fair amount of experience with autocrossing.

Also, the traction issue again. Are you running on true street tires, STS-legal autox tires (ie Kuhmo MX or Falken Azenis), or DOT-legal autox race rubber like Victoracer V700s, 710's, or Hoosiers?

Again, the more traction you have, the more you have a concern with chassis bracing.

Finally, on a FWD car as light as a Civic, only the front strut tower brace is really worth doing IMO. Most of the work (braking, turning) is done by the front tires. You'll see that your front tires get hot after your runs, but the rear tires usually get only 1/2 as warm, if that (indicating they're not doing much other than keeping the body off the ground).

Notice how under hard cornering my EG doesn't even keep the inside rear tire down. That means there's a lot of force on the outside rear, yes - but again, compared to the front, there's not much going on back there.