We installed V7 over a year ago, and I had no problem with accepting the default of 0 for ZPARM ASCSID. However, if you are just installing V7 now, the maintenance may be more finicky about CCSIDs because of V8's implementation of Unicode (just attended an IDUG presentation on V8 Unicode and CCSIDs by Chris Crone of IBM).
In my install guide, Appendix A (Character conversion) gives a table of EBCDIC and ASCII CCSIDs, and it looks like with EBCDIC CCSID of 500, you could ASCII CCSID of 850, 819 or 1252. Granted, for the country of
"International Latin", there are no related ASCII codes, but if you check all other countries using EBCDIC CCSID of 500, you'll see the ASCII CCSIDs that correspond.
But, you may want to place a low priority call to IBM and have them advise you. As the install guide says, once you set these codes, it's not advisable to change them because you can get unpredictable results in a variety of applications (we saw problems in IMS and CICS screens with the wrong CCSID).