I'm getting the feeling that the question of whether to step into signal handlers is orthogonal to single-stepping;
No, it's not. You only ever step into a handler when you ask to. That's already the interface.
Platforms which don't implement PTRACE_SINGLESTEP would probably appreciate this. A "single step" which stops you after setting up the signal trampoline and adjusting the PC, before executing any instructions in the handler.
That's what PTRACE_SINGLESTEP with a nonzero signal number does since it was fixed.