Updated April 21st, 2020.
— Should I update my iPhone to 9.3.4? —The latest iOS is 13.5.1, released Monday June 1st, 2020. If you've installed iOS 13 already you should get the update to 13.5. This is an important security fix. Apple also released iPadOS 13.5.1 June 1st, 2020. If you have an iPhone that can't go past iOS 12, get the 12.4.7 update, released Tuesday, May 20th, 2020.
Hot on the heels of the 9.3.3 upcate comes 9.3.4, and if your iPhone (or iPad) is already on 9.anything you should definitely do the update. The 9.3.4 update contains a security fix that blocks a particular exploit and Apple recommends this fix for all iPhones and iPads.
You can read about the provided fix on Apple’s website.
Apple doesn’t comment on iOS security issues until they have a fix as they don’t want to advertise vulnerabilities before they have plugged the holes. But they do document, at least in general, what they’ve fixed with each update, which does spread the word amongst the bad guy community that certain weaknesses used to exist. That’s bad news for those who don’t update. For this reason I recommend keeping up to date with your iPhone and iOS software unless it’s a major (whole number) upgrade. In that case, I advise updating only twice past the system your device came with. That is, if your iPhone came with iOS 7 installed, updating to iOS 8 and iOS 9 would be good ideas, but updating to the upcoming iOS 10 probably would not.
(The big downside to updating too many times is you eventually end up with a slowed-down device. Each new version of iOS brings new features, and new features take more computing horsepower. There comes a point where your iPhone or iPad will not be up to the task. When that happens, your iOS device will run slowly, and you will wish you could go back… but you can’t. Well, you CAN, but it’s really hard. My “two whole number upgrades past what it came with” rule is a little bit conservative but given how difficult it is to roll back after upgrading it’s the rule I use.)