If you pick up a little iron from your cooking, there is no trouble, as a lot of diets are somewhat low in necessary iron. Don't worry about it.

I found this info on a website:
"Most North Americans don't get enough iron in their diet. If you cook a couple meals a day on iron pans, the small amount that is absorbed into the body by the food represents only about 20% of the recommended daily dosage."

Here's the link

Here is a link that might be useful: cast iron

Unless the acidic food is the main ingredient in your stew the acidity of your stew is probably pretty low. If you have seasoned your cast iron the food isn't coming into contact with the iron anyway. Highly acidic food will take some (or all) of the seasoning off so I wouldn't cook a tomato based pasta sauce in cast iron but I surely wouldn't worry about adding a few tomatoes to a stew.