Almost every site that tests graphics cards uses a high-end dual\quad-core CPU, so as to remove any bottleneck and show what the card is capable of. FYI, most people who are into heavy gaming use at least a 2GHz+ CPU, a dual-core one at that and it's usually OC'd as well. Any Core 2 Duo or AMD X2 CPU running at 2.5-3GHz will not bottleneck any single video card. Thus the reviewers aim for real-life conditions when they test the card. I've heard of people trying to run 8800GTs on single-core Celerons and not getting any performance boost at all. So yes, knowledge about this plays an important role in avoiding it.
Also FYI, bottlenecking can also be caused by slow\insufficient RAM. An example would be running an E8400 with an 8800GS and DDR 333MHz RAM. The RAM's frequency is far lower than that of the CPU's FSB, causing a serious performance penalty. A similar case would be when trying to run a game like Company of Heroes on 512MB of RAM. The HDD would be accessed very frequently, which would show up as stuttering in the game and so the RAM would be a bottleneck in this case, since it cannot hold much information and the game would constantly go to swapfile.