Hey!
They ask that you not enter the parks from backstage, simply for the purpose of satisfying quotas. They always used the example of Magic Kingdom getting filled to capacity around Christmastime, so a bunch of Cast Members going in from backstage can cause problems.
For Animal Kingdom, the cast bus J takes them to the cast bus stop which is in the middle of the parking lot, so everyone walks in through the front. Swipe your Cast ID and go through the turnstiles.
Epcot, cast bus C takes you backstage to the costuming area, where you walk through and take one of 2 other buses. There is a sign there that says which one to take to get to the front gates, I think it's West.
For Magic Kingdom, the A bus makes 2 stops. The first is at the TTC, which is where you need to get off to play in the parks, and then take the Monorail or boat to MK and go in through the turnstiles.
For DHS, the E bus once again has a cast bus stop in the middle of the parking lot, so walk to the front and go in through the turnstiles.
I'm pretty sure the water parks you go in through the front also, I've never taken the cast bus there.
However, you can exit the back ways, if it's easier for you. Magic Kingdom is probably best to do that with, so you aren't in the huge crowds exiting... Just make sure you display your Cast ID around your neck visibly at all times any time you're backstage anywhere.
You can definitely be uncostumed and go backstage to change. Just make sure, like I said, you have your Cast ID visible. I went to play at MK a lot of times before work. Just give yourself enough time to get changed and clocked in!
