I reschedule often because things come up and change with a route. I tend to pick the farthest date out, but then I often end up rescheduling for an earlier pick date. It does not always work out, but it is easier to knock out something sooner than planned and use a reschedule that way as opposed to delays and having to hit that final deadline with no reschedule options offered.
Some clients do allow more than one reschedule. Just don't plan on it. And if you must, try to use the Help link to request a manual reschedule. I have done that a couple of times with good success if I have had issues with traffice delays and hitting time constraints. I try to not make that a habit though. More emails make for more delays in just getting stuff done.