If I just got let go for my job is it in my best interest to sign a

This is essentially an economic, not legal, question, as discussed below.
First, bear in mind that you are under no obligation to sign this agreement; you have to do so voluntarily. This in turn means that they have to make it worth your while to sign it--that have to give you something in exchange for signing, that is enough for you to agree to sign the document.
In signing, you will be giving up any potential claims or lawsuits against the employer such as for employment discrimination. So they have to offer you enough that it is worth it to give up those potential claims. So the issue then is: what claims (if any) might you have?
Employment is employment at will, unless there is a written employment contract. So as a general matter, an employee can be let go at any time, for any reason, and will not have a claim against the employer. So usually, since you probably don't have a viable claim, getting even just a week or two of additional pay as severance makes it worthwhile to sign.
(An employer does not have to pay severance unless it was in a contract--it is volutary for them do to so. Usually, they will only pay if you agree to sign the release--they essentially pay you to sign.)
The only times an employee probably has a viable claim against an employer is if there was an employment contract and the termination was in breach of its--in that case, the employee could sue for the contract breach; or if the employee was fired for a discriminatory reason, the chief ones of which are if he/she was fired due to his/her race, sex, age over 40, disability, national origin, or religion, in which  case there is potential employment discrimination claim.
If you don't think you have a breach of contract or discrimination claim, then  you probably don't have any claim, so if they are offering you *any* additional money to sign, it's most likely worthwhile to do so. 
If you think you may have a breach of contract or discrimination claim, consult with an employment law attorney right away, to evaluate how strong your claim might be and what it might be worth; then you can decide how much additional compensation would be enought to justify giving up that claim and signing the document.