Apple has recommendations on iTunes Connect regarding recommended maximum update frequencies (sooner for bugs that seriously impact users, much less often for minor tweaks).
Don't spam the App store with unnecessary frequent releases; but, at least for marketing reasons, don't let your apps appear to be stale compared to competing apps. You can check on various site (such as appshopper.com, et.al.) on how often other apps in your category are updated, and see how you compare with your app's competition.
You may want to update your app after recompiling and retesting on each new major (or even minor) release of the iOS SDK. Apps that aren't updated, after a new major iOS release becomes the majority version in use, often appear suspect to many potential customers.
The App store update process is not just for bug fixes and new features, but is also required for updating screenshots and some of the important App store metadata, although one does have to bump the app version number when one does this. If you find that your screenshots or keyword choices are hurting your app's popularity and sales, you may be required to submit a no-change bumped version number update to fix this.
You may also want to update some unimportant app just before any deadline for a major app release just to make sure there isn't some glitch or change in the app submission process (new contract requiring a month for the company's lawyer to approve clicking upon, your certificates became moth-eaten, or some such...).