With Asperger's, the trick is always when to explain it (in a non-awkward situation). I'd like to avoid mentioning it if possible, but it has gotten to the point that I only mention it after committing a major social faux pas, in which case it might be seen as an excuse.
With ADD, this is an issue that happens when people explain things to me. I often don't parse things very well when they're communicated to me verbally since I do have attention lapses. That's the major issue, anyways.
With that said, I do (indirectly) imply that I have both diagnoses on my personal website.
This is a difficult situation. If you believe that you will be placed in situations where a someone in question will need to know that you have Asperger's, lest he or she misread your actions in that situation, then you should inform him or her before it becomes a problem. As you said, informing them afterwards can make it seem like an excuse.
However, if your diagnosis affects your "job" performance (academic or research or otherwise), then you should let them know as soon as possible. If there is the possibility to make appropriate accommodations for your condition(s), then you should have this entered into your official records, so that it's available in case it becomes necessary.