Light twins under 6,000 max. gross weight actually don't have to demonstrate a positive rate of climb with the critical engine inoperative. The rule (according to FAR Part 23, certification) is that it has to be determined (usually by demonstration) and the rate of climb OR DESCENT published.
Check the following Advisory Circular for good information on this subject (I used to make this mandatory reading for my multi students back in my CFI days):
http://flight.pr.erau.edu/docs/Aviation%20Reference%20Docs/Other%20FAA%20Documents/01%20Twins.pdf
Page 8 in particular. . .