I can't say that what I did was perfect, but I grounded all the instrument-panel related devices to a common ground bar that was bolted to the radio rack.  Then there is one heavier ground wire that goes to the ground point on the firewall.   It made it relatively simple to manage, reduced the weight of the wiring a little and it seems to work just fine.  High current device grounds should definitely NOT be intermingled with avionics grounds. If you have a "number of different ground points" behind the panel that is probably okay as long as they are tied together with a low resistance connection (heavy wire or the chassis itself).  However, aluminum assemblies are not the best ground systems as the oxide on the metal inhibits electrical connections and consequently there is a risk that some grounds might not be low enough in resistance.  You might think that almost any ground will work for a radio that draws 1 amp, but while the ground might be good enough for that it might not conduct high frequencies well - that takes a very low resistance connection.  Some will probably disagree, but I see no reason that they all have to physically come to a single point.  On the other hand "daisy chaining" connections is likely a bad idea (the ground from one device connects to the ground pin of another device, which in turn is connected to a third device and finally the last in the chain is connected to a chassis ground).