1) If you have them the recommended 2 - 3 ft above ear level, it matters much less.
2) If you have the speakers directly to the sides, it matters a lot more. (I have my surrounds about 7 ft away, but also about 18 in back from horiz.)
3) You also might want to consider bipoles (Def Tech) or Omnipoles (Mirage) too.
I have used dipoles, monopoles, and bipoles/omnipoles. The monopoles were best for multichannel music because the imaging was incredible, but that incredible imaging was a very big minus for movie soundtracks due to the localization of the speaker. Just too distracting for movies. I liked dipoles a lot for movies, but not so great for multichannel music. I have Mirage omnipoles now, and I personally feel that bipoles/omnipoles are the best compromise between imaging for music, and spaciousness for movie soundtracks.
Monopole- standard speakers like satellite, bookshelf, floorstander. Front facing drivers, and all drivers are in phase. Used for main speakers, and surrounds (in a larger room) For example Kef Q1, Kef Q5
Bipole- used as surround speakers.Usually have four drivers. Two treble, two mid, sometimes three sided triangle shape. Both sets of drivers are in phase. Front/back panning is fairly wide, but could still be noticeable. Definitive Technogy BP-2X.
Dipole- used as side speakers. Usually have four drivers. Two treble, two mid. The front facing drivers are in-phase, rear facing drivers are out of phase. Speakers placed at 3 & 9 o clock from the listening position. The area between the sides is the null, which causes the speaker to disappear- location of the actual speaker is hard to localize as the front/back spread is pretty wide. Best used for movies. Some people do not like the directionality of dipoles. Some have exta woofer to add better bass handling. Kef TDM34DS, NHT HDP-2.
Tripole- similar to dipole (back out of phase, front in-phase) however usually a in-phase woofer that faces the listening position. Adds more lack of directionality/diffused sound of dipoles. M&K Tripole surround series.
Omni-poles- a 360' driver spreads sound in all directions. No experience with these, but have heard of some installation issues. Mirage omni-sats.
So here's another question - if he's got a 7.1 setup (I'm thinking about doing the same), what happens when you play 5.1 SACD? Which speakers do the surrounds play out of - the sides or rear? (or both?)
Depends on the pre/pro or receiver. Some you can configure to play the surrounds *or* rears, some play both (but attenuate each 3 dB to get the levels right with the front 3).