You do realize that many houses now have bonus rooms and other such rooms over their garage which typically equates to even more support. Just something to think about. I highly doubt most people will near the 600lb limit.
There is a big difference in the load from snow on a roof compared to the load from a garage ceiling mounted shelf. The roof trusses were design to withstand much more load from the top where the rafters are. The garage ceiling shelves mounts to the roof trusses at the joists, which were not designed for excessive downward load.
600-700 lbs spread across your garage ceiling is no problem, but I would be concerned about 600-700 lbs attached to a couple joists. At minimum, install it near the end of the joist. And be very careful not to put too many holes in the joists.