Be sure to use RFC1918 addresses for the cable modems and all other equipment that does not need routable addresses. It will make your network run better(tm) and will also make ARIN happy if you mention it in your request.
There is no reason to feel, as a requestor, that you need to do things to 'placate ARIN's feelings'. ARIN has no stake in whether operators use RFC1918 address space wherever (in)appropriate. Such utilization has ZERO bearing on whether a requestor is justified for an assignment or additional assignment of address space or the assignment of an AS number. For years I have heard people discuss various things to do to 'make [insert RIR here] happy'. The only thing you, as a requestor or network operator, need to do to have success when dealing with the RIRs is to follow their rules (or be willing to work with them to identify and understand the rules) and not be an unpleasant jerk while corresponding and/or directly talking with them. (The latter doesn't really have a bearing on success directly, but c'mon, with the exception of John Frazier whom I'm convinced is a bot, we're all humans.) /david