The correct answer depends on the version you are using. When I first worked on Calibre xRC in 2004, your workaround would have been absolutely correct. Around 2007, xRC added the capability to do corners with PEX Corner (foundry-supplied) and PEX Corner Custom (for users). These required the newer format rules.
There was a year or two in which xRC worked both with older-style rules, generally in clear text or completely encrypted by the foundry, or the newer-style rules that are always encrypted. About the same time as I was transitioning to the RET products, there was talk of stopping support for the older style rules as they were simply not accurate for nanometer processes. I'm not sure how that all turned out. I do see PEX Corner Custom still in the 2011.2 version of the SVRF manual, however, so presume that is the preferred mechanism for newer releases.
If you would like more help, please provide additional details -- the Calibre version you are using, and whether it is xRC or xACT at a minimum. If you need help with the foundry-provided rules, you may feel more comfortable opening a service request than posting here on the board.