This is an interesting question, one that we talked about a bit in class last night.
I prefer to think that there is no "proper" aikido distance, but there is a proper training distance. What we typically call "proper ma-ai" is the best distance for us to train. Being further away makes the reation too easy -- we have plenty of time to see the attack and respond. Being too close means that we should simply move, blend, and throw, as soon as we feel aggresive intent.
In my style, we use a one-step striking distance for our initial ma-ai. The proper distance of connection then varies depending on the technique that we do. For example, with irimi nage, we tend to be very close, ikkyo is often done more at arm's length.
Think of the training ma-ai as the ideal distance, but work with what you have. When we do randori (multiple, simultaneous attackers using any attack), ma-ai can vary considerably. Sometimes everyone is right on top of me, sometimes, I open two or three (or more) steps of space between everyone else and me. When people are too close, I try to open up space, when people are too far away, I try to close the space by entering strongly.