Filters and reality

I'm wondering just how far someone can get ignoring the prefix-length
filtering rules used by Sprint/ANS/etc as long as you use Spring/ANS/etc
as your provider.

The National Archives of Canada has a traditional class B network 142.42.
However, instead of advertising the network as 142.42/16, they've elected
to advertise only 142.42.242/24. The /24 prefix-length announcement in
traditional class B network space wouldn't pass the access-lists some
providers use on their inbound BGP sessions. The same providers don't
seem to use the same filters on their outbound announcements, leading
to customer (and often network engineer) confusion why things don't work
to some places but work ok inside the providers own network.

Should we continue to poke holes in our filters to let Sprint's customer's
customer routes through, or should we keep trying to explain to the Sprint
NOC why their customer's customers could use shorter prefix length

Be liberal in what you send, and conservative in what you accept?