Copying information from an online tree is a great way to repeat incorrect information. Just because something appears in an online compilation–even with a submitter name–does not mean that it is correct.
The goal of your tree should not be to see how many names you can collect. Treat researching that distant relative you just learned about the way you would a close relative–try to be as accurate as possible. If finding incorrect information on your grandma would upset you, keep in mind that person you just “copied and pasted” into your tree with out a second of validation may be someone’s grandma as well.