On my single bike with a double I always put the chain on the smallest front ring and smallest rear cog, 39/12, and take enough off so the upper pulley doesn't contact the chain.
With the triple on my Santana if I put it in the 30/12 , and yes I know not to use this combo, I could add a little more length to the what came from Santana.
What is YOUR method?
I think a little more length wouldn't hurt as long as there isn't any rubbing of the upper pulley or am I wrong.
First bike with a triple.

I follow Sheldon's advice.

"The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that they could be connected (outer link meets inner link), then make the chain one complete link (one inch) longer than that. In almost all cases, this will give the optimum length."

+1 for Sheldon.

Reason is that it matters little if the deraileur doesn't take up all the slack in the chain, but bad stuff happens if you try to stretch the chain. If this happens, either the wheel will pull sideways, the chainring will fold up or the deraileur / dropout will pop. For this reason it's a good idea to rigorously test the installation in all gears in the workstand and have more rather than less chain.

Always use the big / big combination, through the derailleur or as described on Sheldon's web site.

Bad things happen when there's not enough chain to go around the big /big combination and tandems invariably end up crossed-up in the big / big combination. We've had to unlock a few locked-up drive trains on new tandems where the bike shop short-changed the chain. Thankfully, these tandems had self-extracting crank bolts that allowed us to pull the drive side crankset, albeit in an ugly way.

The little / little is of little consequence since it's basically an un-useable gear combination for a variety of reasons, e.g., chain drag on the derailleur and chain chatter on the middle ring all begin about mid-way down the cassette.

I use Big-Big and always run it through the pulleys. Then just pull until it is minimal, and go 1 longer. If it doesn't work on small-small after that, then your derailleur doesn't have enough takeup. And I agree with TG that tandems seem to be attracted to bad combinations, but I am getting better at anticipating when we really are going to need the next smaller chainring, and try to get into it before we run out of cogs on the back.

One advantage of the Shimano Flight Deck computer is the graphic gear display, which might help you keep from considering a shift into the large-large combination, because you will know when you are on the adjacent cog.