If you need to keep it secret or not, you still have to encrypt and obfuscate, just to protect yourself from random hackers.
On the other hand, I do not believe that you can stop a particular hacker. The combination of jailbreak, gdb and traffic sniffer will hit almost any protection you can think of. Investing in such protection rarely makes sense, so you will need to find a trade-off between wasting time and effort and using your API key.
API , , App Store, . ptrace() PT_DENY_ATTACH ( ) gdb. , , . HTTPS , .
. HTTPS , API HTTP-, . , .