why go against REL setup recommendation?

Thanks for all the responses. I do plan on listening and finding what works best in my situation. It was the exact debate I'm seeing here that caused me to ask the question.

Best to start in the corner, since REL recommends it, even if it does not work in the end. If it does work as intended, the corner is also usually good from a standpoint that the sub is out of the way of the system and other things in your room. Put a plant on it :-)

I would like to add my perosonal "Summiko trainning" experience with all interested here, regarding proper speaker/sub placment proceedures....
...Let me just say that I've personally had the expeirce of being instructed by Summiko regarding proper setup proceedures for speaker placment in a room in the past. Unless Summiko's theories and theachings have changed over the years, I really don't see how anyone could accptablly and consintently get good results by following their recommendations!
Without getting too long winded about what I heard and measured from Summiko's set up in our store, I will say that the measureable and audible results of their work didn't pass mustard regarding proper speaker location and fundamentally sound, reasonably flat/even frequency response from their set up! (infact I think none of the speakers in their set up that I measured (again, from their listening possition they established) yieled such acceptable frequency response, and certainly there wasn't adequatly EVEN OR sIMILAR frequency response among ALL THE SPEAKRS!!!...needless to say, I wasn't impressed, nor do I accept their theories. But while they did offer some sound advice regarding speaker "toe-in" and "aim" for proper tonality and immaging from their speakers, and maybe proper soundstage width and scope, and related theories, I don't likely aggree with how they would recommending you approach
PLACING speaker in a room in general.
Now I'm sure Summiko knows their product, and it's abilities/limitations, and perhaps Rel's sub does best in terms of acceptable bass output while placed in a corner...I don't know. In fact corner placment might be the way to go in a lot of cases where the sub hasn't enough output for the room/system it's in...this is often the case with a lot of systems, where the room requires more sub(s) to balance the sound out properly, and provice enough weight. What happens in a lot of cases is people don't have enough sub for the room/system it's in, and the sub needs to be place in a corner to reinforce and help boost the bass output in the room..basically "under-subing" if you will. IN that case, I can wholeheartedly recommend that for alot of subwoofer applications. It would probably be much better, without having to heavily "EQ" the sub out, to place one ore more subs in other locations in the room, where you're getting more "even" response from your sub/system, and also having "enough" sub to handle the job. And if you do have enough sub(s) for the room/system it's in, I believe other placment considerations are going to get you more even, balanced, and smoother results sonically.
IN the end, it's all balance, and the only thing that matters is results!
I might ad however, that corner placment is the most likely candidate for NECESSARY "EQ'ing"! The corner spot is going to excite ALL the bass modes in a given room, more than likely anhwere else! The sound is going to have the propensity for being peaky and boomy, and will lack definition and overall accuracy of sound...giving an unatural and/or "small-room-boom" kind of sound. In this case, I STRONLY RECOMMEND the incorporation of a good parmetric EQ into the subs path to take down the peaks in the room(from the woofer that is). The results this way can end up being very satisfying overall if done effectively.
Never-the-less, any sub setup where you are going to end up with the reasonably acceptable flat and even frequency response from your sub(s) in a speaker system in terms of location in a room, achieve adequate and accptable output and volume, and still maintain proper phase in relation to the other speakers in the system, is going to be a good one!
Results are everything!...theory is just theory in the end...

I had my Strata III in the corner and now it is between my Maggie 3.6's. Much better. The dealer set it up in the corner and it sounded pretty good, but I moved it, he came back to drop off some cable and he agreed that the sound was better with the sub where I had it. Bottom line is that you've GOT to move it around and adjust until the sub fits your room/speakers/taste.

Generally speaking, I find it interesting when people say the subwoofer "sounds better" here or there. A properly integrated subwoofer for 2 channel audio is sonically invisible. It should not call attention to itself.

The specific recommendation is in the corner behind one of the main speakers. I suppose if your speakers are out in the room, or part way along the longer wall, then the sub placement may change also.

A lot of REL's market is outside the US, and mostly only us American have these large dedicated media / listening rooms. The III series REL's are very adjustable. You should be able to adjust the sub to the chosen placement in most cases, instead of adjusting the sub by moving the placement.

Many folks have only one place for the sub in a multi-purpose room, or those with WAF placement issues.