I don't know the answer to your question.
I suspect no issues, as many veggies are staked using metal stakes or supports around them , or planted near chainlink or other metal type fencing, and I have never heard of any issues about that. If you paint or otherwise coat the metal that may make a difference.
But have you considered using natural rock or preformed concrete retaining stones? That way you don't have to worry about this issue, they won't rust like steel would, and you could always change/move the layout in a different way later on if you change your mind about the look of it. I have also found that either stones or concrete holds some resale value if you decide later to nix the whole thing. It would look more natural as well. Depending on what you are talking about regarding the metal I think that would be some very custom work and could get quite pricey. If that is the case in your situation, buying the stones may prove to be more cost effective.
