Great question, John! To answer your question with regard to citing information you find at NEHGS: if you include information in a wikitree profile that you found in an online record at americanancestors, you should cite the source.
A good practice is to simply copy-and-paste the citation information from the NEHGS result into the wikitree profile. The citation information at NEHGS can be found on (most) result pages by clicking the "Citation Information" link in the area above the image/text.
In addition to the citation, I suggest you include other specific information such as the volume and page number from the navigation bar above the image, and, if applicable, the publication date, the title of the article or document. etc.
The New England Historical and Genealogical Register. Boston, MA: New England Historic Genealogical Society, 1847-. (Online database: AmericanAncestors.org, New England Historic Genealogical Society, 2001-2013.) Vol 17, p 12, Jan 1863. "Brief Memoir of Dr. Winslow Lewis."
Not only will that help you to comply with the use of copyrighted material, it provides invaluable information that will help you and others, in future, locate the exact information you used to draw your conclusion, review the data, etc. As you apparently know, and it can't be repeated too much, including source information in wikitree profiles is not only good wikitree practice, but it provides a means for the future extension of families (the ultimate wikitree goal), documenting research, resolving disputes regarding names, dates, relationships, etc.
If you care to, using the citation example above, go to the advanced search on americanancestors; select "New England Historical and Genealogical Register" in the Database dropdown; select "17" in the Volume dropdown; enter "12" for the page number; and click the Search button at the upper right or lower left. Select one of the results and you'll see the same information cited.