The couple times I've been above 280, I have. I guess the controllers need to know so they can still provide 2,000 feet separation for you since you're an outsider! Also, according to the FAR/AIM (2008) you should attach "non rvsm" to the items below. (page 152, table 4-6-1)
Pilot will report Non-RVSM status as follows:
a. On the initial call on any frequency in the RVSM airspace and....
b. In all requests for flight level changes pertaining to flight levels within the RVSM airspace and
c. In all read backs to flight level clearances pertaining to flight levels within the RVSM airspace and
d. In read back of flight level clearances involving climb and descent through RVSM airspace (FL 290-410)