The decode also converts PCs to symbols, so for all the data to be useful you have to boot the exact same kernel
With the same modules loaded at the same addresses (if your traces are going to include paths through loaded modules).
Perhaps include a hash of the kernel symbol table so the decoder can tell whether it has any hope of making sense of the trace? [Compute the hash at boot and when modules are loaded/unloaded, then store in a header section of the binary ftrace record]
-Tony