asterisk users - Dec 2007 - Asterisk Realtime SIP rtcachefriends

I haven't been able to find this on the wiki:

If rtcachefriends=yes.  When will a change to a realtime user/peer take effect? 
Next registration?  Never?

It's also not clear to me what the purposes of rtautoclear and
ignoreregexpire are.  The only info I have found is the comments in the sample
config file.  Sounds like rtautoclear will save memory if I have lots of peers. 
Is there any other reason to enable it?

And why would I want to ignore the fact that a registration is expired?  It
seems if registration is expired and they don't reregister then the SIP
device is probably not there anymore. Correct?