I'm developing an app supporting since iOS 3.0. With Xcode 4.2, I have to add armv6 manually to Architectures. So is armv7 optimized for new devices the only advantage to compile with "armv6 armv7"? The size will be bigger than with only armv6 ?
Using only armv6 is not optimal - but it will work on older iOS devices (i.e. no defect).
An alternative is to provide both armv6 and armv7 binaries so newer devices will use the armv7 code. Of course that will double the size of the application code. It might be worth it depending on your application (e.g. if speed is an issue) but it will not affect compatibility.