in software engineering should a contract review take place if a proposal was properly prepared and approved? please give advantages and disadvantages of both cases.
A review is a review. The point of review comes because you have yourself put the question "if a proposal was properly prepared and approved?". Does this question return a true value always? How do you know that the proposal was PROPERLY prepared and does not have any doubtful clauses? Next, business strategy, market condition might change over time. So contract review is just a way to make sure whether "a proposal was properly prepared and approved." and necessarily should be there. If the reputation or confidence on the "proposal preparation" is high, the rigour of the review will be less and depends upon the experience of the reviewer.
If this is a commercial, business question then SO isn't really the right place to ask.
I would, however, say that as with any business transaction the devil is in the details.
You should always review the contract, in particular, intellectual property rights (who owns them and when), payment details (to whom, when, for what, how soon, what currency), penalties, schedules, deliverables (precisely what and in what form), maintenance and lurking pitfalls (such as no limit on liability if your product should cause your customer financial loss or personal injury).
The proposal is really an addendum to the contract, in that it deals with the functional aspects of your code, but the contract is what covers the financial and legal obligations of both parties.