Negative Gs are when the train is dropping faster or tighter than a parabolic trajectory would cause. It pushes you up as you want to continue along your path and the coaster drags you down with it. Technically, it's 0G. Now, floating begins when you get below 1G and can be called airtime without any major quibbling.
By their OTSR nature, Inverteds [and standups, and loopers...] tend to have little to no air, but try to overcome in in other ways. Non-loopers [woodies, hypers, freeformers, mice, etc] try to overcome a lack in elements with either more interesting forces, or a profusion of laterals or airtime. Woodies, however, can benefit themselves by racing through their structure or using headchoppers. Airtime, however, is often the force du jour for nostalgists and woodie-fans. It's a force more unique to this styleof coaster than laterals and the like, and is thus more valued.
Plus, it usually hurts less than laterals. =)