Most zoom lenses that have a "Macro" designation tend to only have a maximum magnification ratio of around 1:3 to 1:2 at the longest zoom range; they will likely not deliver the same level of performance at this point as a true fixed focal length macro lens. [IMAGE'S LINK: http://www.flickr.com/photos/mrsbuckley/2690678995/]
On the other hand, these lenses sometimes have some advantages; they are generally less expensive than fixed focal length macro lenses, and they have a longer focal length than most macro lenses; this, and the 1:2 magnification offered by some 70-300 zoom lenses, means that they are good lenses where added working distance is an advantage (for example, insect and dragonfly photography). And, of course, they have infinity focus, which you would lose with stacking lenses or using extension tubes (infinity focus is available with most prime macro lenses, with the Canon MP-E 65mm as a notable exception.)
As an example, here is a dragonfly I took at 300mm (about 3 feet of working distance) and my Sigma 70-300mm APO DG lens and 40D:
The picture is sharp enough that I can see the texture of the eyes on the 100% crop, but depending on your subjects, the image quality afforded by a proper macro lens may be necessary.
Extension tubes coupled with a 50mm prime lens are also an option that may give you high magnification as well, but my experience has been that there is also a very visible cost in terms of edge image quality when using these, you must get very close (1-2 inches) to your subject, and luxuries such as autofocus and infinity focus are also nonexistent.
To make a long story short, telephoto zoom lenses with "macro" designations are a good way to start in Macro photography, but they are by far not the best way to do it.