A quick search provided this copy of the NSPE Code of Ethics...
"4. Engineers shall act for each employer or client as faithful agents or trustees."
I think it is more important to first consider your ethics and not just because they are often adopted into state law in some form or other...
Not too many years ago we started our business.
I would say, DO NOT solicit your employers clients while you are still collecting a pay check. If you haven't signed a non-compete agreement then in most cases you would be free to contact these clients after your end date. Use the Yellow-pages and don't keep any documents that your employer hasn't authorized or have knowledge of... You will likely find that you are going to do it your way anyway, it is about you now and who you are going to be as an engineer/entity.
Always have an honest and positive based response to any questions that may arise regarding your departure. I finally started to say something like, "Well you know how it is, we were like Teenagers, we thought we had all of the answers and had to give it a try. [chuckle]" "[ex-employer], was always a really good boss (mine was), blah blah blah" be honest and don't embellish, forget about anything you didn't like and never talk about it if they bring it up...
Be prepared for unpredictable receptions... Some of these clients that you felt really close to were close to you as a part of your bosses company. Others don't feel such a loyalty to your employer and you may overlook them because you don't have a glowing recollection of them. We found that relationships were really important, those based on trust even more so and usually already taken... Some that were initially turned off or even cold, later remembered who we were and gave us a try, but we share their work load with our past employer. Don't forget to look up the clients currently with your bosses competition, there is always someone on the bottom of the list, it takes time to build your own client base.
I worry that the image of the Engineer doesn't have quite the bang it may have had 50 years ago, I try hard not to contribute to any further erosion. In the end only you know many of the truths and it sure makes it easier to sleep at night when you didn't do anything unethical. Good luck and let us know how it works out...