Hello Joe,
A general rule of thumb is to wait 6 to 8 hours after midnight in the
time zone local to your account before requesting a basic report for
the previous day. This will give the Reports Center enough time to
process the majority of your clicks and impression data for a given
day.
As for determining when a report will have data that is absolutely
final, no, there is no way to do that via the AdWords API. But this is
not just something that applies to reports scheduled via the AdWords
API--reports scheduled via the AdWords front end are functionality
identical, and the same limitations apply.
I have a feeling that you'll hear about the experiences of some other
AdWords API developers who have faced a similar decision about report
scheduling in this thread, but since this is an issue that applies to
AdWords users in general, you can also post about it in the general
AdWords discussion forum at
http://www.google.com/support/forum/p/AdWords
Cheers,
-Jeff Posnick, AdWords API Team