You can usually get in if you're not picky about the time, but I would at least call and make a ps after you get to WDW. If you're going at a super busy time (say Easter week), you might need to get one ahead.
We have frequently made our ps while we're driving to WDW. The only one we've ever not been able to get is an in-park character breakfast. As long as you're somewhat flexible, you can usually get in. I do like to go to restaurant with a ps, though b/c it really helps you to get seated more quickly.