Even after the Flight Readiness Review officially sets the launch date, the liftoff can get scrubbed at the last minute (or, in the case of STS-134, at the 225th minute). You can schedule your travel to allow for a one- or two-day scrub, but then you might find out that it's been punted back a full week.
If you'll be coming from far off, you might want to see if you can't telecommute from the greater KSC area for a couple of weeks. If, OTOH, you're lucky enough to live somewhere with cheap flights to Orlando, wait until FRR to book--and then either get a no-change-fee or refundable fare or get a one-way ticket, buying the flight home once the shuttle launches.
IOW, it's kind of a pain to see a shuttle launch in person. And (speaking from my experience watching Endeavour lift off Monday from the press site) it's worth every bit of the hassle.