The easy answer is do not do it. When you excessively water before aeration the ground is to soft and you will pull more dirt than expected. Now when should you do it? I would like to say never but if you have a really bumpy lawn you can do it if you plan on leveling the lawn afterwards. Along as you plan to add soil and seed. If you do not add those items then you could create patches of dirt in your grass. 

   Now if your soil is compacted to the point where water is constantly running off the top then a tiller would more effective or soil conditioner. After doing those actions then you could aerate later down the road.