online tools and wikipedia are not bad at the moment. Who tells that this will remain so? Organizations like wikipedia needs money and there is no long term guarantee that not at one point in the future, a "sponsor" will jump in, and searches or articles will be "internally vetted". Like for anything, it is good to have many independent sources. And yes, I keep as many copies of old encyclopedias and handbooks as possible, so that if needed, things can be double checked. Even in math software (like computer algebra systems) it can be healthy from time to time to check results with old handbooks or other systems, like integration tables. And of course be able to look up the original sources or reproduce things yourself. Never trust one source alone.