When using multiple imagery sources you should aim to align them at ground level which makes them easier to work with, features like fences and walls tend to be the best references for this; they have clear edges and are close to the ground.

Regarding project 12803 you are requested to offset the imagery to align with map data becasue it's a long lived project and the imagery has changed. When this happens the position of features may move because their position is claimed to be within X m of the true location. 
 it isn't worth moving all the existing data; the objective is for all features to be accurately aligned relative to one another.

Though few contributors do it, adding imagery offsets you used in tasking manager comments is highly recommended. It's beneficial for subsequent contributors becasue they can be coppied, and it also shows if you're correctly offsetting imagery. The best way to do this is to copy and paste the offset you used into the comment box as soon as you've determined it. There is no need to add the offset to changeset comments becasue when the imagery changes it likely won't be valid anymore. I figure it likely wouldn't cause any harm, but so far I haven't seen anyone add offsets to changeset comments. I stated a few times which source was the one I was aligning to but currently I jsut let the order of the sources imply that information: in JOSM I tend to have my primary source in layer 1.