Gain staging is nothing new to the audio world of course. Most analog emulating plugins and/or convolution types also need to adopt the gain staging practice for proper handling of the signal. -18 dbFS is a widely accepted, varying to some degree as others have said, depending on the gear/developer. In the end though, while keeping the reference in mind as your guide, you should always still adjust according to your ears and using a good peak meter as your ultimate guide. Real world signals are complex and depending on harmonic information, dynamics, transients, and the like you can't heavily rely on RMS metering for example. Peaks dictate where your signals are; they lead. So if you are looking at a preset that needs -18 for example, in some specific case, it may turn out that -20 is better for it as your ears tell you and clip lights and the peak meter.