YAY! Congrats on gettin' her on the back!! I'm sure she had a blast (and it probably won't be long till she wants her
own bike!)
The placement of hands on the tank versus wrapping arms around the pilot depends on a lot of things (the bike, the size of the pilot, the height/arm length of the passenger) but mainly boils down to personal preference. I spent over a year strictly as a passenger, and found hands placed on the tank was the most comfortable all around... BUT, I'm tall, and have long arms, so it was easy to do without putting pressure on the pilot, and without clinking helmets. As Rob mentioned, this is also a very effective positition when hard braking comes into play.
Here's a couple threads on other forums that might help... have your wife read them -- there's lots of good tips that should help her get a bit more comfortable until she finds the position that works best for her.
Good luck!!
Tips for Riding on the back
Ridin' on da back....
Oh yeah, and DO invest in a good pair of gloves for her (if you haven't already).... Something with a non-skid (like suede) palm, or even gel pads, to reduce fatigue.