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How important and what is too much?


It's relatively important.
Generally, there can never be too much.

From a design and tuning point of view (in other words both in theory and practice) one wants a chassis that has effectively inconsequental compliance relative to the suspension system.

As usual what one does on specific vehicle will come down to projected tires and application. The mopar chassis was fairly decent. But even there, notice the factory decided to make various improvements or additions. If you are going to autocross or do track days on R compound tires, then definately add at least all the factory extra bracing. Also, check all the welds whenever possible. For example if the K frame is out, check it over -reinforce and repair seams or spots (within the liitations of rules you'll be running under of course).

How much flex depends on the chassis/body style and the loads input. The cross braces in the back of my coupe eventually tore from the floor pan. Was that due to poor assmebly alone? I suspect that was only a contributor, the other part of the equation being increased spring rates - bad or unpaved roads and lots of autocrosses on R comps. OTH Mitch autocrossed his Challenger very competitively with more tire with no problems that I'm aware of. At the time, SCCA Street Prepared rules didn't even allow welded frame connectors (now that's allowed with restriction on attachment points). On the other hand, anyone running a nationally competitve C Prepared car has pretty much braced the heck out of it for very good reason.

Herb Adams's Chassis book has a good pictorial overview of effective and ineffective bracing.