I wrote a document that describes why we need justification and sort of mirrors ARIN's form (sample available on request). I've inserted it into the sales process - nothing moves forward until we have a signed contract and the IP form (without it, we default to /29s). And us IP provisioning guys are such a hard ass about it that sales knows not to bother assigning a ticket over unless they have that form.

During contract renewals we nmap their address space and if it doesn't meet our requirements we ask for justification again. At this point they either justify and we like or we change netmask or customer forks over more money. The latter seems to encourage one other first two options.

And in some cases additional /24s are so "cost prohibitive" that the customer's encouraged to plead their case to ARIN and get their own space.