Our 767s default to 12 but we overtype this with 4, obviously because we are RNP 4 'capable'. Because RNP 10 doesn't really exist (it being RNAV 10) there is not actually a requirement to enter 10 in the FMC, since alerting is obviously not required for RNAV, only RNP, but I've seen guys do it and I've got not problem with that since it's not actually doing any harm to have the ANP monitored. I have always wondered the same thing why it defaults to 12 when it's RNAV/RNP 10 but I just shut up and drive the thing instead
Edited to add: when I say RNP 10 doesn't really exist I'm talking Japanese (JCAB) world, whereas reality may vary quite considerably.