Opinion on that differ, both between riders and which version you're using. Some have reported acceptable performance straight off when mixing Campy 10 and Shimano 10 once they've found the right chain to use.
One option is to use a device called a shift mate. It looks much like the Travel Agent and it changes the amount of cable pull to get a perfect match.
Another option is to get a custom cassette that use the spacers from one system together with the sprockets from another.
Here's a good site:
http://www.ctc.org.uk/DesktopDefault...o%2011-24-2005