No, not really. #pragma pack and/or __attribute__((packed)) have been supported for eons now and guarantee the alignment of struct members between compilers.
In newer C++ specs, you can also static assert that the struct is a POD type to statically ensure that there's no accidental vtable pointer.
This argument pops up every time someone mentions this and every time it's completely uninformed.
Though it should be noted that packed structures cause compilers to produce absolutely garbage code when accessing them (because most of the accesses become unaligned) and it becomes incredibly memory-unsafe (as in "your program may crash or corrupt memory") to take pointers of fields inside the struct because they are (usually) presumed to be aligned by the compiler.
Explicit alignment doesn't suffer from this problem nearly as badly (yeah, you might have to add some padding but that's hardly the end of the world -- and if you have explicit padding fields you can reuse them in the future).
In newer C++ specs, you can also static assert that the struct is a POD type to statically ensure that there's no accidental vtable pointer.
This argument pops up every time someone mentions this and every time it's completely uninformed.