IMO in-wall is generally preferable, but depends greatly on the quality of the speaker. For rear surrounds it probably isn't as big a deal.
THX recommends this for what is basically a 6 channel setup, with 2 identical rear speakers side by side. They aren't trying to localize sounds between them, and only recomend two to avoid a so-called "reversing effect", where in some cases sounds coming from behind you can seem to be coming from in front of you (or so they say). For Dolby PLIIx or any kind of surround mode like Logic7 from what I've read it's best to space them farther apart, to achieve true stereo separation (this may also apply to future Blu-ray or HD DVD discs with true 7.1 sound somewhere down the road). I don't really know of a good reason to place them close togther; from what I've read you'd be better off to place them farther apart, to achieve separation.