...if people call for 'no warning for slash' then there should also be a 'no warning for het' call

Yeah, I think "warning" would be listing a warning, as you might for a kink or something like non-con. Informing people that there will be sexing involving x pairing in the fic is just basic info, like story length.

So if I see Jack/Rose/Nine and and adult rating, I'm assuming I'm getting het and slash action (Probably. Or it could be a tossup in that case.)