On a new build you will be given a certificate of compliance with planning and building regulations on closing and probably a structural guarantee of some kind.
So you will have something to fall back on if things go wrong.
However:
a. Your bank ( assuming you are getting a mortgage) will insist on a valuer giving a certificate on closing in any case, and may have a list of approved valuers which can include both engineers and auctioneers. If you are going to get an engineer, you could go for one approved by your bank on the basis that he/she would be doing the valuation for the bank anyway, so might give you a reduction on inspection for you. The banks valuation will not be any guarantee structurally, so an inspection solely for the bank will merely be superficial and nothing to do with structure.
b. If you do get your own engineer to inspect on completion, they will probably also do a snag list for you.
c. You will have the peace of mind that your own independant engineer has inspected and you will also be able to rely on their report and insurance if anything goes wrong.
d. It has been known for new houses to have substantial structural defects- for eg one estate I remember had such problems that the blockwork had to be torn down and done again. Caught by a purchasers engineer.
e. On the other hand you may decide to rely on the builders guarantee and their engineers certificate. If so, it would be much better if you had a friend who had some knowledge of building to inspect for you at least.
Up to you.