In my opinion, "use jQuery" is not a valid answer any time the asker has not specifically asked about jQuery, and here's why. The user should have to opt-in to hearing about jQuery, not opt out.

Google is a great tool. Anybody interested in finding about about javascript libraries can easily do so, and there are a lot of great ones out there. But directing every question about how to do something into an answer about how to do it in your favorite library is redirecting what could be a good, general purpose answer that anybody could use, into one that is only useful by people who are able to use your library. Rampant evangelism notwithstanding, there are a lot of projects in which using jQuery is outside the scope and requirements of the project. The project might require using a different library or no library at all, and the library chosen if any might not factor into the wording of the problem. The user might simply be interested in knowing how to do it in plain javascript due to the fact that certain things are really hard or impossible to do properly unless you know what browser or even version of the browser or OS you are using. jQuery might abstract this away from you, but in certain cases this is undesirable, and I would argue it helps keep programmers complacent when they should be rising up and calling for better interoperability between browsers.

It seems like 90% of the time I am trying to find out how to do something in Javascript all the answers I find are how to do it in jQuery. As a result, I either have to dig into the jQuery source code (not gonna happen, I've tried it, and I just don't have the time) or ask my own question specifically requesting how to do it in plain javascript. Now I have wasted my time and everybody else's repeating a question that was already answered, but in such a way that it is not useful to me.

Having said that, if you feel the need to tell the user about jQuery, I have no problem with you supplementing your answer of how to do it in Javascript with how to do that in jQuery. Now you have not only educated the user how to do what they wanted in plain Javascript, but you have also shown them how much easier it is in jQuery, and they can make a more informed decision as to whether they want to start using it.