I've searched the forums, and the variations of this question are not often specific enough to get a solid answer, so here's my situation and question;
I have a solid answer for you.
How could that go wrong:
1.- The hard drive dies, this way you could lose the BTC
2.- Someone gets physical access to your mac and dump the PrivateKeys.
3.- You get hacked. (In some of those short 20min access to the web.)
In the end, there is no 100% secure way to hold btc, but i will recommend you to take a look to paper wallets. You can use programs like VanityGen to create an addy and private key, then put that info in a text file and the file inside zip with a password. That way your bitcoins are secured by a password. Or at least get a physical wallet like Ledger if you want to feel more secure.