I have some assembly routines that are called by and take arguments from C functions. Right now, I'm assuming those arguments are passed on the stack in cdecl order. Is that a fair assumption to make?
Would a compiler (GCC) detect this and make sure the arguments are passed correctly, or should I manually go and declare them cdecl? If so, will that attribute still hold if I specify a higher optimisation level?
This is a matter of the ABI for the platform you're writing code for. Almost all platforms follow the Unix System V ABI for C calling convention and other ABI issues, which includes both a general ABI (gABI) document detailing the common ABI characteristics across all CPU architectures, and a processor-specific ABI (psABI) document specific to the particular CPU architecture/family. When it comes to x86, this matches what you refer to as "cdecl". So from a practical standpoint, x86 assembly meant to be called from C should be written to assume "cdecl". Basically the only exception to the universality of this calling convention is Windows API functions, which use their own nonstandard "stdcall" calling convention due to legacy Win16 dll thunk compatibility issues; nonetheless, the "default" calling convention on x86 Windows is still "cdecl".
A more important concern when writing asm to be called from C is whether symbol names should be prefixed with an underscore or not. This varies widely between platforms, with the general trend being that ELF-based platforms don't use the prefix, and most other platforms do...