I am wondering whether it is necessary to replace various bushings within the suspension?

 

certainly not necessary, but replacing bushings with poly bushings (Energy suspension) will help you car's handling. A few of them are pretty easy (motor mounts, front sway bar) and most of the others require more extensive work.

 

I'm still waiting for my ES bushings to come in so I can chime in on how much diff the car will feel. Having rode in a few low mileage Maximas mine feels more loose than any of them even with my JICs. I've had my car slammed for 3.5 years now at over 2" so I am sure my bushings are very shot at 112K miles. I just got the ES controll arm bushings for now, trailering arms for the rear beam will have to wait- as its a big job.