There shouldn't be any reason that stairs can't be used while placing in grid mode and yet they can't. This is incredibly frustrating when trying to build an elaborate exhibit or food area.
If there is an underground already then I don't need the auto-tunnel in the first place I was trying to build a ramp down from surface level and then back up again. The up was easy, the down not so much. I had to manually dig it out and even then I ended up with some wierd bump in the ramp that looks stupid though doesn't appear to affect the guests any.