all [0s|1]'s subnets

I keep running into people (Ascend tech support, joeblow ISP tech support off the street)
that say that it's improper to use the all 0s or all 1's subnets (ie .0 and whatever
has .255 in it). Since CIDR, I thought the philosophy was 'use everything', but I can't
seem to find the explicit recommendation to start using the .0 subnet... can anyone
help with a pointer to the pertinent RFC ?