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I've always been an OAG man simply because it eliminates a lot of unnecessary cost, fiddling and heartache. I used to use an OAG with my st8300 with my TMB/lomo 80/480 years ago and never ever had problems. Essentially I was guiding at 1 as/p and imaging @ 2 as/p so even a considerably large guide error of 1.5 full pixels only translated to <1 pixel of drift in the image. No flexure, no fiddly adjustable rings, no large difference in scale and it removes the need for extra heaters, rings, extension tubes etc that all cost money and add weight to the mount.
That said Fred vanderhaven guided his 12" SCT at f10 with 6.8 micron pixels using a piggyback ST80 and 7.4 micron pixels. I don't think there are any hard and fast rules. Your results will dictate what you need to do.
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