Q. Do I need to include a retrieval date for all of the sources that I find online?

This is a common question, and the answer is very different depending on the citation style that you are using:

APA Style

According to the APA guidelines, you normally do not need to mention when you accessed a source in your citations. Include the retrieval date only when you are citing a source that is updated frequently, for example a Wikipedia article (although really you shouldn't be citing Wikipedia!). 

• When to Include Retrieval Dates for Online Sources
  This article from the APA Style Blog gives more information on when to include retrieval dates.

MLA Style

The eighth edition of the MLA Handbook does not require that you include a date of access—the date on which you consulted a work—when you cite an online work from a reliable, stable source. However, you may include an access date as an optional element if it will be useful to others. (See the MLA Handbook, eighth edition, pp. 50–53, for more on optional elements.)  

Chicago Style

The Chicago Manual takes a similar approach to APA Style, noting that the value of the access date is of limited value as it cannot be verified by the reader. Chicago does not require the access date "unless no date of publication or revision can be determined from the source" (section 14.7 of the Manual).