Hi Derek,
Adding a validateRequest="false" is in my opinion not a good idea as it imposes the risk of allowing users to "inject" Html directly as you can see from this article here.
In some cases, this might be necessary to work around an issue that can otherwise not be solved, but I would strongly advice against it if it is at all possible to solve using the method desribed in the article referenced above, by using HtmlEncode.
If you really need to stop the validation, then I suggest you do that as locally as possible, meaning that you should only stop the validation of the exact controls that need it and not globally for the application.
I am very curious to know on which control(s) you do get this error and if you have tried to eliminate it ?
Palli