It depends on the pattern.
Some molding patterns cannot be coped since there is not enough wood present at overhangs (the pattern is undercut).
These patterns are typically expensive and large though, since the require angleing cutters to produce the shape.

How about a style with "dental teeth" plus the standard curves. Is that typically coped? Like this:
http://images.google.com/imgres?imgurl=http://www.crownmoulding.com/images/mouldings/self_return.jpg&imgrefurl=http://www.crownmoulding.com/cutting.htm&h=268&w=472&sz=12&hl=en&start=4&tbnid=KaosezNuYQNWvM:&tbnh=73&tbnw=129&prev=/images%3Fq%3Dcrown%2Bmolding%26gbv%3D2%26svnum%3D10%26hl%3Den%26sa%3DG

How about this one (if it were one-piece foam):
http://www.ehlkitchens.com/images/accessories/dental%20crown%20molding.JPG

There's no reason a dentil crown couldn't be coped, unless like brick said it has undercut curves.
I lay out dentil crown so the _outside_ mitered corners form perfect cubes at the dentil blocks, and let the inside corners fall where they may. If an inside corner ends up awkward, the gap in the "teeth" gets some filler or wood "dutchman". A lot of dentil crown comes with the dentils as a separate strip to be fitted afterwards. In the very best work, the "teeth" are laid out individually with varied spacing so that inside and outside miters would correspond to full blocks (nobody does this any more, except a few fanatics).
Casey

Thank you. Can you clarify what you mean by undercut curves?

Is that a curve that reverses vertical direction, like this:
http://www.traditional-building.com/article/images/beak.gif

(I found it in this interesting article:
http://www.traditional-building.com/article/moldings.htm)

Thanks again.