Thanks dadt - I'd half-suspected that the figure/prop distinction might have something to do with it. And if the important criteria for PhilC's statement was that "...Constrained is only for where the cloth is to be constrained against a portion of a figure*, but NOT a prop)*." then it fits.

But I'd ruled that out because (1) in a very old (2002) PoserPros Constrained vs choreographed cloth (nudity) tutorial wdupre (Will Dupre I assume?) stated that "...constrained groups are the part of the fabric that is rigidly attached to a figure or prop by parenting the cloth.", and (2) some of the vertices in my clothified curtain did stick to my rod prop.

However, a lot's changed since 2002, and I suppose some fluke could be causing some of my curtain vertices to stick to the prop. 

I'll try adding a bone just to prove to myself that what you've said is correct.

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).