Not necessarily. Scientists have a hard enough time pinning down exactly what all the impacts of cattle grazing are (and furthermore extrapolating them into "good" or "bad").
Sheep grazing in the U.S. saw it's heyday in the late 1800s and early 1900s. You may have heard about "range wars" during those times. Often those "wars" were fought between cattle ranchers and sheep ranchers. Ultimately the cattle ranchers won out.
Well it wasn't until just a few decades ago that ecological impacts were really much of a factor in our land management decisions. So we don't have much scientific data from the days of widespread sheep grazing to compare to our contemporary data on cattle grazing.
Historic sheep grazing has been blamed for the disruption of ecosystems across the western U.S. It could be argued, though, that the destruction had more to do with poor management than the destructive nature of the animal. For example in 1880, 4 million sheep were recorded just in the state of New Mexico. Compare that to 2005, where the total count for the states of Colorado, Montana, South Dakota, Utah, and Wyoming was 2.65 million.
Sheep graze in a different manner than cattle. For example, they bite a plant off lower to the ground, and they prefer some different species of plants (depending on local ecology). Furthermore, they don't disturb the ground as much, due to their smaller mass. If in one area long enough, or faced with limited forage, they will eat nearly every green thing in sight - which is exactly what occured in the historic herds of thousands across the western us. They would leave a devastated wasteland in their wake, as the shepherds moved them along.
The biggest problem with sheep grazing, in my humble opinion, is the sheer number of animals. Sheep farmers enounter problems of economy of scale when considering grazing on public rangelands.
If you're interested in viewing some of my sources or reading into the topic more:
Fleischner, Thomas L. "Ecological Costs of Livestock Grazing in Western North America" Conservation Biology, Vol. 8, No. 3. (Sep., 1994), pp. 629-644.
Stable URL: http://links.jstor.org/sici?sici=0888-8 ... 0.CO%3B2-N
Denevan, William M. "Livestock Numbers in Nineteenth-Century New Mexico, and the Problem of Gullying in the Southwest" Annals of the Association of American Geographers, Vol. 57, No. 4. (Dec., 1967), pp. 691-703.
Stable URL: http://links.jstor.org/sici?sici=0004-5 ... 0.CO%3B2-7
http://www.nass.usda.gov/wy/internet/ra ... rr0508.pdf
http://www.cebc.bangor.ac.uk/Documents/ ... razing.pdf
What did the parasitic Candiru fish say when it finally found a host? - - "Urethra!!"