How accurate is "inferred states"?

Mmmm I don't like using inferred state because someone could be traveling, etc and that would be listed in their lead record.

I'd use the actual state field, I know this won't be populated for everyone though. You can def clean the lists many different ways - if you're looking to format (ex: all Florida, FL > just be FL consistently), or fill in state fields based on area codes or postal codes. This is all lots of work but can be done via smart campaigns.

Inferred fields are not 100% accurate because of travel, regional ISP hubs, etc.

BUT they can be generally helpful for countries and State when those are not available from the lead or a data appending service.

One thing you can do is use a service like reachforce and then you do not have to ask for state.  They will try to find the correct location for you.  There are several services like this and can be very useful.