machineKey validationKey="AutoGenerate". How bad is it?

machineKey validationKey="AutoGenerate". How bad is it?

config looks like
<machineKey validationKey="AutoGenerate" validation="SHA1" />
I'm trying to figure out how bad is having a database of already 15,000+
users using an AutoGenerated machine key and MySQLMembershipProvider.
What's the machineKey uses?, or better, where is stored? if I change the
server (physically) or doing a clean install, will users be able to login
?
I tried to read lot of documentations but couldn't actually get a good
article or description if Membership uses machineKey or not or if it's
only for ViewState, SessionState, etcState.
Currently I have no problems, i just wondering if I need to take any
consideration in case of moving servers, etc. My real concern is because
of not having a machineKey value users may lose the ability to login with
their current credentials.
If it's recommended to use a non auto-generated key (why?). How can I get
the current one so I can specify it and rest safe or can I just drop
random one and everything will still work?