Never breath in when receiving an attack, always breath out!
Good advice, this is what I see all the combat Ki and chigun masters do when they demonstrate. Didn't harry hudini (spelling?) die of an unprepared blow to the stomach?
Recently in our dojo we've been placing a lot more emphasis on atemi. We never actually talk about it too much but sensei is having us make sure we do it alot on our entering motions for certain techniques. At my particular level of aiki ability, I find it very hard to get certain techniques to work well without atemi.
It is most definitely a necesity to aikido training.
Have you guys considered trying out those padded suits that karateka train with sometimes? Would be seriously hard for uke to move in one of them but you would be able to see if your strikes are effective.