There are no rules as such – it's always the end result
However, you're right to consider the analogue equivalent as a good
starting point for Nebula chains, and of course the tracking stage can be extremely
Many synths/drums/guitars/basses/vocals may have been recorded
extremely cleanly during the tracking phase, so you can add some extra 'character' to
taste in the form of preamps/vintage tubes etc. at the mixing stage, as well as any
console and tape emulations that you fancy.
Remember that many Nebula presets
can be subtle, particularly the consoles, and only really come into their own once they
have been added to each channel as they would be in the analogue world.
Personally I'd prefer the flexibility of adding such character to the mix rather than
committing to it at the tracking phase, but unless you've got a massively powerful
computer you may well have to add all this Nebula finesse as a rendering process rather
than hearing it in real time.
Hope this helps!