Later in the game when catapults are rolling and alliances are at war, the worst thing you can do is slam your main offensive forces into a huge anvil. If you are building a WW hammer, save it for the WW! Safely raiding farms with your troops is fine, but build a second (smaller) hammer for alliance warfare. Here are some tips to protect your "disposable" hammer and get the most out of it:
1. Fakes! Send dozens of attacks with 20 troops and a catapult. This will disguise your real attack. If you are sending multiple waves, ensure each of your fakes has multiple waves. If these extra troops divide the defenses, they are worth far more than their cost to produce. Get your alliance mates involved and time the fakes to arrive near the same landing time so defenses can't be re-positioned.
2. Don't go for the jugular. An attack on a secondary or tertiary target is much more likely to be assumed to be a fake. These attacks can be devastating and your hammer will come home in tact to be used again and again.
3. Watch for the follow-home. Good players will often calculate the arrival time back at your village and send a swift cavalry force to wipe it out. Make sure you stack your defenses in advance of your troops arrival so that you are not left scrambling.