I NEVER merge from any tree or gedcom. However if I find the image of a document, I will place it on my hard drive and source it to the owner of the tree on which I find the document, I also try to find the original person who posted the image, which sometime is difficult. I don't know how many times I have found chain references for sources, John attached from Joe, Joe attached it form Sally, Sally attached it from Jane, etc. The net results is the data is un sourced.
If I find information of interest, I first contact the owner and ask about the information. From experience, only about 5% of the owners are confident of, or have any knowledge about the information to respond.
If the information is very interesting I added it to the notes section of my database. I put it in a note attached to the person to which it applies, clearly marked with the confidence in the data. When I have verifying documentation, I will then added it to my database. To me verification is, at a MINIMUM, a document image, or a defined transcription traceable to the original documents.
When I started working in genealogy I did some merging, 10 years later and many hours trying to clean the garbage sources from my database, I am still finding questionable sources from those merges.
I hate the way Ancestry handles sources on their trees. Even though they have listed "sources" many of them are circular, and are worthless.