It really depends on what you're doing. If you're creating HTML5+CSS+JS for a web/PhoneGap app, it will probably be OK as initial test (unless you encounter bugs caused by PhoneGap itself). The reason is that underneath both the Android "Internet" browser and iOS' Safari, there is a WebKit core, which is somewhat cross-platform and mature enough.
If, on the other hand, you're writing native code, then don't trust the simulator/emulator. There is a myriad of things that can go wrong. An iOS Simulator example that has bitten me: iOS is case-sensitive. Mac OS X is case-preserving*. Guess what happens when you request a file from the iOS device that is in the wrong case. Tip:
EXC_BAD_ACCESS. App - Rejected! (Of course, you should test on a real device way before sending the app for review, so did I, and there wasn't really a rejection).
I can't even start to enumerate the differences, and I'm certain Android emulators do have inconsistencies with the real devices, too. It's just a too complex system to work smoothly on a non-native CPU and in another host OS.
Simulators and emulators do not exist to replace testing devices, they exist to let you reduce testing time for things you'll test later more thoroughly on the devices itself and (possibly) to allow you to get a basic understanding of the framework without paying for the test.
The moral of the story is: don't trust simulators/emulators, they'll turn on you the moment you start depending on them. And from what I've seen, differences are usually very subtle, so finding them and debugging the app is hell.