If you don't, you'll forget something. Have a back-up plan that matches the importance of the session. Bear in mind that backup preparation can require a lot of time. Simple backups can work well. A quick diskette copy of your presentation as a backup can come in very handy if you somehow destroy the copy that's already on your computer.
Handouts are good things to provide in any presentation, but they can be useful backups too.
If everything falls apart, you may still be able to save the presentation by teaching from the paper document. I'm astonished at how cavalier presenters are about technology at conferences when so much can go wrong. Common disasters include projection resolution that doesn't match the presenter's laptop resolution, or a presenter not knowing how to split the projection signal so it can be seen from both laptop and projection screen simultaneously. Ask to have technical backup available on an on-call basis. In our courses, we specify in our contracts that tech support will be available within 5 minutes of a call.