No rules really, experiment and go with what your ears like.

Personally I mentally map it like this:

First I go for a scoring stage sound using the regular reverb.  I more or less focus this set-up around the Brass, and imagine I'm standing on the podium in front of the orchestra in a 'rehearsal hall'.  I only want enough reverb to let the harmonics of the brass instruments meld a bit and come across as being more natural.  Typically I go with shorter reverb times, but I saturate it pretty good with higher levels from the brass, and very little if any from the rest of the orchestra.

Next I try to get things more or less panned in space where I want them to live.

Finally I use the Convolution reverb to build a 'room'.  This is where I go for the longer 2 second and higher reverb times.  At this point I'll gradually start cranking up the reverb sends for the Woodwinds and everything else that's not Brass.  I keep tweaking individual sends, and the master level of the two reverb units until I find a decent and somewhat natural sounding room that blends the brass over-tones with everything else.