When your site is ready to go live, you'll want to take inventory of which feature tests you used, and reduce your Modernizr JS library to include only those tests that you need. This way it is as small as possible, downloading and executing more quickly than it would with unused feature detections present and firing.
The Modernizr module has you covered; it keeps track of all the tests being requested by other modules and themes — just visit
admin/config/development/modernizr. There are three sources: modules, themes, and tests within your current copy of modernizr.js.