I understand the importance of timing, but if they are waiting until Mueller delivers a knockout punch then it is far too late. I would think this is the perfect time to drop their bombshell and completely discredit Mueller and bury the Democrats after the shutdown of the government. If they decide to release this information after something more significant from Mueller and his team, they look like the kid who got caught punching his sibling telling his mom "well he hit me too!"
Additionally, if you allow a buzz to build and the report is nothing more than the suspicions of Nunes, which may not be corroborated by anyone else, that may be even worse! If this memo is as explosive as they are saying, the GOP has a small window to act and it is closing very quickly in my opinion.