Looking in an index is not a search of the original records. So, I search in Ancestry.com and record that I looked in Ohio, Deaths, 1908-1932, 1938-2007 and do not find my ancestor. Does that mean I am finished with that record? What if the database is updated by Ancestry.com in the future and I don't know about the update? Considering that Ancestry.com's collection is an index extracted from more than 4 million birth, baptism, and christening records from Ohio, how do your know that your ancestor wasn't skipped by those doing the indexing? Or even more likely, how do we know that the person doing the index didn't mis-transcribe the information? But realistically, are we really going to search through 4 million individual records looking for our ancestors?
So it appears to me that recording where we look is valuable but not conclusive. Reviewing or re-searching records, especially those that are searched by using search engines or indexes may be fruitful from time to time. If that is the case, then what is the main value of a Research Log? No matter how prolific your memory, you will not remember where and what you have looked at after the passage of some time. Without a record of where you have searched, you will find yourself repeating the research. As I indicated at the beginning, this may not be all bad, but can be frustrating.
A more fundamental reason for recording where we search is to give us an indication of the limits of our previous searches. Hopefully, as we progress in our genealogical knowledge, we come to realize the vast number of additional types of records we could have searched initially but were unaware of their existence. As we go back and look at our Research Log, we may start to understand that we could have used many more records at the time.
When I mention a "Research Log," am I talking about a paper form you fill out with the places you look? Yes, it could be a form and in fact there are number of different forms for that purpose available online. But realistically, I am really referring to the notes and lists you can keep in your genealogical database. Every major commercially available genealogy program provides for a way to store your notes on an individual and/or family. Many programs provide a to do list capability and some have the flexibility to design your own reports including Research Logs. There is no "one way" to keep such information. You have to evolve and adapt each method of recording information to your own needs and style of doing research. But basic to the entire idea of doing research is recording your sources and that includes the places your research that do not contain specific information as well as those that do.
The key here however is to remember the transitory and ever changing nature of Internet research. You may need to dig down a bit more than is comfortable to find the original source and not rely so heavily on indexes, abstracts, summaries and other online finding aids as conclusive evidence of whether or not your ancestors are recorded in the original record.