Is it a bad idea to buy plane tickets before the conference schedule is announced? I will be attending a conference to give a presentation
in approximately two months.
I would like to buy plane tickets to and from the conference location,
especially because I am living outside the US
and the conference is in the US,
so travel to the conference is not an easy thing.

Unfortunately, the conference organizers
have yet to confirm and announce the conference schedule.
I do know that the last possible presentation session ends at 2 pm on Tuesday,
so if I wanted to book plane tickets now,
the earliest flight that I could catch would leave at 4 pm.
However, if my presentation is on Monday,
I might prefer to book a flight that leaves early on Tuesday morning,
skipping some of the sessions at the conference.

Question:
Should I book my flight tickets now at "safe" times?
Or should I book my flight tickets,
leaving early on the last day,
hoping that I don't get scheduled for the very last presentation slot?

Response to answers/comments:
I don't need to leave early.
I have just often observed that
at every conference which I have attended in the past,
the attendance at the last few sessions is rather low,
and it just seemed to be "normal" to leave early.
Based on the answers/comments,
it seems that just because many others leave early
doesn't mean that I need to when I can stay at the conference,
learning things and meeting people.
First, I would ask whether you think you really should skip part of the conference---you're going such a long way, and presumably it's a community that you care about, so are you sure you want to abandon part of it?

If you really do want to skip, though, I would recommend a third path that I have often seen work: contact the conference program chairs, let them know your travel dilemma, and ask them if they can schedule you on the prior day.  If they are likely to be flexible, this will let them be flexible now and let you book; if not, you'll know they're unlikely to be flexible at other times either and thus know that it's not safe to plan to skip until the schedule is announced.