Rubber mulch has become a somewhat popular mulch medium due to it's "forever" qualities". By the same token, because it lasts "forever" it is not as beneficial to plants as organic mulch such as wood-chips. Organic mulch bio-degrades which adds organic matter and nutrients to the soil. Organic mulch also does a better job of moderation soil temperature and conserving soil moisture.
To answer the question of rubber mulch retarding growth... no, I have not heard or seen this to be the case. It's just that it's not quite a beneficial to a plant or planting as organic mulch.
If you haven't already run across this resource, you may find it helpful. http://pubs.ext.vt.edu/426/426-724/426-724.html