Hi, Mary --
While it's not forbidden to add a post-1700 profile with a source of just "unsourced family tree," it would be even better if you could add a more reliable source like a census record.
I like to use FamilySearch when I create a new profile because it's usually pretty quick to find at least one source.
If you can do that, it's going to give the next person who encounters that profile a lot more to work with, and they won't have to wonder if the data you provide on the profile is accurate. Our goal is to add accurate, verifiable information.
Our 'thons are fun ways to improve our tree, and it's important that while we're trying to make a lot of progress, we also focus on quality research. :-)
(And I have no idea what your genealogical expertise is, but that was a great question, and it gives us an opportunity to share that info with everyone. Thanks!)