Gustaf, if your code relies on API or Includes from a third package a dependency is in order. Also make sure that you update the dependency requirements from time to time.
All packages should have a dependency on the kernel version they need to run so you could detect what version of OpenACS is needed.
So better be save then sorry and add more than less strict dependencies to the system. I realized a couple of times that I forgot to update the required version number, which left me sometimes wondering in the dark why it worked on site one but not on site two.
