If I want to configure a Hidden Service but only for personal use and didn't give out its hostname/.onion address to anyone. Can its address still be crawled/known to any third party?
Summary: If no one has a Hidden Service address, then no one will be able to find it and so I don't need to worry about configuring as if its a publicly accessible service?
If you really want a hidden service to be private, you need to use authenticated hidden services. On the server side, this is the HiddenServiceAuthorizeClient options, and clients will then need to add the HidServAuth option to their local configuration. Access to the hidden service will be impossible unless clients have both the address and a valid authentication cookie.
The downside is that it's pretty cumbersome for the clients as there is no integration with the Tor Browser currently.
No, it's not private. Hidden services publish contact information (descriptor) to hidden service directories. One of them might be evil, and use the data in a search engine. Also, methods for identifying hidden services have been published, and vulnerabilities have been patched.1,2 But maybe others haven't.
1. Biryukov et al. (2013) Trawling for Tor Hidden Services: Detection, Measurement, Deanonymization
2. Jansen et al. 2014 The Sniper Attack: Anonymously Deanonymizing and Disabling the Tor Network
