You shouldn't need on if it didn't come with one. If you notice an exhaust leak there then you can get some silicone or teflon gasket/sealant from your local bike shop and that will fix it right up. Hope that helps.
The popping sound you hear when you're slowing down or letting off the throttle is normal. Happens because your fuel injection system isn't exact to the akrapovic exhaust you got. If the popping sound bothers you that much, you can get a power commander (for fuel injected bikes) or a jetkit (for carborated bikes). Not really necessary for just a slip-on, but a must for full systems. Some still like getting a power commander for a slip-on. Up to you on that one.
I don't have one, and I've been riding fine for a couple thousand miles...
No I can definately deal with it. Its really no that bad. Just wanted to make sure it wasn't because of an exhaust leak. In other words I just wanted to hear someone say it was normal. Need to get the Dr X fender eliminator kit before I buy a power commander