I have found a bug in Plants vs Zombies 2. Do I need to report it to them?
A user is never obligated to report bugs, but doing so should always be highly encouraged. Developers can't fix what they don't know is broken. :)
That being said, the best thing to do if you're having problems is to start here: Plants vs. Zombies 2 Support
You'll see a bullet titled "Basic Troubleshooting for iOS Games" that you can click for some really basic advice on how to get your game working. It is worth noting that if you intend to report a bug, you should include as much information as possible, and the most important information is how the bug occurred and whether or not it is repeatable. You should consider following their instructions and then attempting to reproduce the bug before clicking the Contact Us link provided.
Even if you don't have the time or capability to do such in-depth bug testing before submitting your information, it is still worth it to tell them what you know as it has the potential to improve the game for everyone (i.e. reward = knowing that you may have improved the lives of millions of fellow gamers).
Any game developer will accept bug reports, some have specific bug tracking systems open to the public, others have no specific policy and accept e-mails. Popcap does not seem to have a public bug tracking system, you can contact them here. There is no guarantee they will acknowledged your report.
Most likely there is no reward, as the bug may have been identified internally already and is awaiting a patch rollout. Software companies generally don't reveal who caught the bug first for liability/creditability purposes - as this may expose weaknesses in their internal quality assurance mechanisms (however some do).
Many companies have specific policies against exploiting bugs. So you can keep the bug to yourself but exploiting it may incur a penalty.