You need to identify risks and base your approach around that, the advice you'll get from a UX forum will typically lack acknowledgement of some of the internal workings and edge-cases (superficial, if you like).
I assume you are talking mobile, since you mention push-notification device registration.
Why are you providing a sign-out feature to the User? (please answer that, I am always interested to know exactly why)
If receipt of push-notifications after an attempted logout continues, and those contain private or sensitive information = a risk!
If a phone is sold or loaned after an attempted logout that sees another person able to subsequently take control of the account = a risk! Really, everyone should be factory-resetting before selling, so user education is the way to tackle that (as is done with Phishing). Also Apple and Google provide Lost Phone features, those exist to mitigate against the stolen/lost phone scenario and do it a lot better than expecting the user to pre-empt the unfortunate situation by aggressively logging out of your system.
My advice is, careful consider your rationale for providing a sign-out feature in the first place, if you decide it is important then make sure it works. That means retrying when a user attempts it but it fails, local Notifications can inform the User of the problem but the system shouldn't leave it up to the user to manage the problem (why write software and read UX forums if you can't auto-retry on behalf of the user?)