TAINT NECESSARILY SO
If I have heard it one time I have heard it a million times and I wish I had a penny for everytime I have heard it.
"Well, it must be true, I saw it on Ancestry.com/Familytreemaker/LDS/some website/a book."
Just because it is on one of the various websites, doesn't make it a fact. The data on those sites is submitted by other researchers just like you. Or perhaps worse, not just like you. You don't know just how conscientious they are in their sources or their work.
Unless you can contact them and get a source and then verify it, you should never, ever just assume something because it was on the website. Try entering data on one of those sites, they don't ask for any documentation, you could enter anything you choose to enter.
Many times I have attempted to contact persons to obtain sources and never hear back from them.
I found one on one of my ancestors that has proven a hard case to crack..she listed his place of birth as Dearborn county, NJ. And like a dummy I looked it up, KNOWING that there was no Dearborn county and never had been. I never heard back from her when I wrote for her source info. That data was from a source that most persons consider very reliable; the LDS.
Unless you are dealing with a site that can show you actual images of census, military records, land patents, or immigration info (such as the Ellis Island site); then don't believe it for a moment until you verify it.
The County History books are also in question as most of them were done on a type of subscription basis. Meaning that persons of influence or making donations to pay for the printing were normally given a better reading then others. If the others were covered at all.
This is not to say that they were all incorrect, but just be aware of the source materials and document everything you can before accepting it as fact.