My take on stems for touring is that you should try and set up your bike for all-day comfort. The stem can raise or lower your bars, and it can also extend or shorten your reach to the bars (from the saddle.) I like the top of my bars level with or only slightly lower than the top of my saddle. I've found this lessens hand soreness/numbness. I have my seatpost pulled out quite a ways for my long legs, so I like a stem with some rise to it, to match. I can't tell you what gauge I use to determine how far I like the reach from the saddle to the bars. I just ride and play with it until I find a setup that feels right. You can do this by sliding the saddle forwards or backwards, and if you can't get it right you can also try a different stem with less or more reach.

The reach on my LHT seems rather long, so I have a stem with lots of rise and not much reach.

I bought a few stems (Ebay helped) before I found the one that seemed right. I kept them and they were usefull in setting up my next bike. I'll probably keep them, in case I or one of my family gets a new bike in the future. It's nice to have a few different models to try when you're setting things up. Even if you don't settle on one, they can help steer you to a purchase that will work.