To do negotiation or not really depends on how good is your negotiation skills and how good/bad the offer is.
Some company says their first offer is the best offer, no more negotiation. Most of the companies will leave a space in there and anticipate you to negotiate with them.
Better do negotiation face to face or on the phone. Do your home work, be well prepared, knowing what to say. Have Plan A, Plan B and Plan C. The bottom line is to keep the offer, unless you have other offers.
Start the talk in a nice professional way. They will respect you more once they realized you requirement is reasonable. Your goal is try to create win/win situation for you and for the company. You get what you want, company make you happy and they do not need repeat the long going hiring process again.
Please don’t be greedy. The hiring manager does withdraw offer if he/she feels you are asking for too high and meaningless.
There are friends who did successfully to negotiate salary. But, before you want to negotiate, think about the following questions:
--What will you do if they do not reply your email upto the deadline of the offer?
--How do you know they will not meet what you want?
--What will you do if they can't meet what you asked in your email?
Communication can solve all the problem. You need to communicate with them anyway.
Finally, if the offer is at-will, you can reject it even after you signed to accept it. You make the decision based on what is best for you and your family. Rejecting an offer is very easy. However, don't simply reject the offer before you even have a chance to negotiate with the company you want to join most, also don't resign from your current job too early.