Just speaking generally, I usually ask myself these questions when I encounter someone with far-out views:
1) Is this new behavior for them?
2) Am I actually responsible for their behavior?
3) Could this behavior hurt them or someone else?
Since MIL has always been like this, and being your MIL you might reasonably feel somewhat responsible for her, that leaves #3. If she believes Obama is literally the Biblical Anti-Christ (actual theory from someone I know), her believing that probably isn't going to hurt anyone. If she feels his presidency means the End of Days is nigh and she ought to spend all her money before the world ends, that could harm her and her descendents, and someone should step in.
Regarding the dementia aspect, what I imagine is that the health professionals monitoring her would ask the family if this was new behavior for her, and when they said no, they would exempt it from the list of mental illness symptoms. So to me, this doesn't really rise to the level of what she says harming her.
If people around her think she's daffy for saying these things... well, she's 90 years old and in her right mind, she's probably gotten bad reactions before and has decided that it's important to her to keep on saying these things, despite the perception. In other words, if she was really concerned about people thinking she was daffy, she would stop saying non-mainstream things at all (as long as she has the mental faculties to make that decision).
We had a thread recently about ways to beandip relatives with wacky, often offensive conspiracy theories; it had some good ideas about how to redirect the conversation, if what she's saying upsets other people. There's certainly no need to let her dominate the conversation with topics that are hurtful; someone can swoop in and change the subject, without subtlety if necessary.