the standard is to route all instruments through a small number of effect inserts/auxes, though if your pc setup will allow it, I'd suggest setting each instrument's reverb separately, though by and large you'll want to use the same type of reverb for each (e.g. a concert hall or club setting).
that way you can fully adjust the dry/wet mix ratio and any other settings to best suit each instrument separately.
if you leave your instruments all in the middle you'll have to turn the gain (volume) down on each instrument to avoid overloading your output (speakers/headphones) just because all the frequencies are stacked up in the middle ... making all the instruments sound undiscernably quiet.
not only does panning's let you place your instruments in the L-R stereo range much as you'd expecto to hear them live (or any other arrangement you'd like to hear) ... but it also spreads the frequencies/volumes out more so you can afford to keep each instrument at a higher level right across the L-R spread
adjusting the dry/wet mix ratio for each instrument helps you place instruments instruments in the front-back audio range as well. use the wet fader to enrichen the instruments sound, and then adjusting the dry fader makes the instrument seem closer or further away. finally adjust the overall gain to bring the volume up/down to balance the other instruments
at the same time you might want to have a look at using multi outputs on your VST (Kontakt or Aria I presume?)
sorry I can't tell you how to do any of that in cubase though!
there are a lot of helpful tutorials both on the threads on this forum and on the Garritan website ... but they can be a bit hard to find.