IIRC, computers/electronics are not among items that will be credited if your baggage goes missing. If you can afford it going missing or have insurance, than you can take a risk, but I am never confident that a five-finger discount will not take place, even with a TSA lock.

Personally, my computer would not leave my side, not even gate checking on RJs.

I am not sure which iMac size you have, but I would really recommend seeing if you can fit one into a carry-on size luggage, if so, bring it on board. i have seen people carry desktops and large monitors aboard.

I would be more confident that Fedex will get it to your destination.

I think the only situation where I would feel comfortable checking desktops would be if you can "saran wrap" the baggage and ensure it arrives like that. I have seen this at international airports, but I am not sure how TSA would deal with this.