I've seen that on other forums as well and don't know where they were getting that idea from. Makes no sense to me at all. Maaaaybe on the original SE Soapbar, but I highly doubt it. Definitely not the case on the II or the One I have.
The trouble you can run into, at least with the Soapbar II, is the neck pickup rout is shallow. Not a problem with regular P-90's, but any stacked coil design is too tall, side by side designs are iffy. About .75" is the max pickup depth that'll work, and that's pushing it.
I don't know if that applies to Singlecuts, though. They have a thicker body, so the rout may be deeper.