That might be true but having a massive formation of hundreds to thousands of planes was pretty hard to hide. The Germans had a sophisticated radar network that detected the bomber stream before it crossed the coast. The contrails might have helped the crews spot attacking fighters sooner, although they might have partially obscurred them, too. Hard to say. From what I've read and learned from discussions with WWII bomber crewmembers, they had a lot of things to be afraid of. Flak was probably the most dangerous thing they faced. Towards the end of the war, the Germans had a hard time mounting much fighter opposition to bomber attacks but flak continued to be a deadly threat the whole time.