Can you give us a little more detail on your wife's level of skiing and in what conditions she typically skis? (East Coast hard-packed, or West Coast powder, etc.?)
Even without that info, I'm leaning towards recommending the riser plate be mounted. If your wife is striving to become advanced, she'll start to carve more, and having a riser plate helps a lot on parabolic skis. When I got my first pair of parabolics, some old K2 Threes, they did not have a riser plate. Occasionally, on steeper slopes at faster speeds, I would 'boot out,' as jdistefa says. I would be on my edges at such an angle that I ended up on the side of my boots...and as you can imagine, that would end up in me skidding out and having a nice big yard sale. Once I installed risers, however, that extra height below the sole gave me a few more degrees on my edge, preventing any sort of boot-out.
Though you say your wife doesn't ski the steeps, it's possible to boot-out like that–even on blues–if you're on enough of an edge. I personally can't think of any major disadvantages of having the riser installed, but perhaps others more knowledgeable on this forum can put in their $.02. For that reason, however, I would say that it won't hurt her to have the riser installed, but it could hurt her to NOT have it installed, even if it's not until later on...get my drift?