Task Killers: As much as this is obvious, people still uses it. So much, that it is annoying. Android handles apps and tasks well. However, there is an exception, and that is when the app constantly freezes or crashes and you need to restart it. If that's the case, there's a "Running Apps" section, where you can force close apps under system settings. So you won't need 3rd party Task Killers, EVER
Battery Calibration (Draining then fully charging): People thinks that this is good for your battery life, but it isn't. The only thing that this does is make the system read the battery percentage more accurate. Doing this once per month might help you improve read battery % better, but it isn't necessarily at all.
Wiping battery stat when the phone doesn't charge up to 100%: This myth is quite annoying. I dunno about other forums, but in Galaxy Nexus forum, quite a few people use this myth as a answer. Wiping battery stats DOES NOTHING to how system reads battery%.
Anti-Virus app: This is another myth that is obvious, but used by many people. You don't need anti-virus apps, unless you download a lot from untrusted source (4shared, because it is uploaded by people, not the company). If you know (And you SHOULD) what you're downloading, anti-virus is nothing.
Benchmark: People get so hyped about benchmark results... And I dunno why... It's just a number, I tell them. It wouldn't matter in daily usage, I tell them. But they do it anyways... Benchmarks are just numbers people. My Gnex can be as smooth and fast as GS3, but have a significant difference in benchmark tests.
Quad-core is faster than dual-core: While this isn't a myth, it isn't 100% correct either. You see, in daily usage, the phone only uses and needs a little bit of CPU power. For example, Facebook app doesn't use full 4 cores. That's just plain silly. However, when it comes to heavy gaming, yes, that 2-extra cores will help. But overall in daily usage, you're not gonna miss almost anything, because you have dual-core...
You need to clear cache and dalvik when updating/flashing a custom kernel: This is not true. I flash Franco kernel without clearing anything with NO problem.
You need to fix permission whenever you dirty flash: I don't hear that too often, but people still believes in it. Fixing permission isn't necessarily, unless you're experiencing random force close or crash.
Turn GPS off when not using: You do not need to do that at all. Phone only turns on the GPS when it needs to. For example, when you open map app, you'll see the GPS icon on the status bar, but when you exit out, you won't see it anymore. This means that the GPS radio is off. OFF. No need to waste your time turning it off and on.
Switch to 2G (Edge) when screen off to save battery: This is probably one of the biggest mistake people do to save battery. Yes, 2G does consume less battery than 3G, but constantly switching between them just burns the battery. It is just pointless if you ask me. Either stick with 3G, or stick with 2G. Constantly switching between them will be the worst choice.
Undervolt as much as you can to save battery: This is not true. Undervolting to a certain point can help you extend your battery life, but doing it too much will drain more battery instead of extending it. Also it makes the phone less stable, so make sure you know what you're doing and don't undervolt like crazy
iOS apps are more optimized than Android, and is more stable: This is part true but mainly not. Yes, dev only needs to worry about 2-3 screen size when making apps, which means that apps can be optimized for the screen size, but that doesn't mean much anyways... Also did you know that iOS apps crash more than Android? Now you know
iOS is better than Android : Lol wut?
Hope that cleared few things up. Again, if you got a myth of your own, feel free to post it in the thread!