In my opinion, no, unless you planned to drill into and plant into the rocks themselves (a specialized use which would normally require softer sedimentary rocks such as tufa or other porous limestone that provide a porosity network for the roots to grow into).
The rocks are really secondary to the growing medium, which needs to provide good drainage to grow a lot of what we consider rock garden or alpine plants. The rocks can provide structure, height, and aesthetic properties to the construction, but a gravel or sand bed with no rocks at all can grow plants just as well. To demonstrate this point, you can browse through umpteen pages on the construction and fabulous successes of Tim Ingram's sand beds on the SRGC forum:
http://www.srgc.net/forum/inde...
For other great examples of sand/gravel beds, look up entries from Anne Spiegel (she mostly talks about crevice beds but she has every imaginable sort of rock garden bed) and Bob Nold on the NARGS and SRGC forums, e.g.:
https://www.nargs.org/forum/wh...
Back to your point - rounded "river rock" cobbles and boulders can be used just as effectively as angular broken rock pieces. There is an example of a "glacial moraine" bed built by Cohan at NARGS using cobbles that I think looks great - I'll try to find it and post a link.
