@Jim Brozny
After a claim your insurance company will give you the actual cash value of the damaged property which is generally much less than the total cost to replace. If you have replacement cost coverage they will pay the remainder of the cost after the job is complete and you send them proof of the total cost in the form of an invoice from the contractor. So you will not have the opportunity to pocket any money. However, if somehow you do get the opportunity keep in mind that if you are caught committing insurance fraud in Illinois of between 300 and 10k you will get to enjoy a minimum two year sentence in prison.
You will not need to worry about doing any of the negotiating yourself either. Generally the insurance company will inspect the damage and come up with some $ figure they think is fair for the work. The contractor can sometimes negotiate with the insurance company if they believe it will cost more then the initial offer. It is often beneficial to have your contractor and the adjuster inspect the property at the same time so they can talk and agree on a price.
Remember, the insurance company is the one making the payment so they have incentive to negotiate a fair cost. A large portion of a roofing or siding contractors business comes from insurance claim related work so these contractors have an incentive to develop a good relationship with the insurance companies.
That being said, it is still very important that you properly vet your contractor like you would for any job. The one big difference in the vetting process after a claim when you have replacement cost insurance is that you do not need to worry about price since it is already determined by the insurance company.
@Jeff B.'s advice to get three bids is absolutely correct for normal work. However, after an insurance claim instead of getting three bids you could conduct three contractor interviews to determine who you think is best for the job. If you already know and trust one contractor you can also feel free to use him.
It is still very important to make sure you have a quality contract and have hired someone capable of finishing the job. My opinion is that you should hire a large well capitalized contractor with years of experience. These large contractors often have standing contracts with insurance companies that they will accept the price generated by so and so estimator program making the process totally seamless.