Exposed Pad on a mixed signal IC, ground plane connection and power supply decoupling
It really depends on how many layers your  board will have. 

If you have more than two layers, it will almost always be better to drop vias down to the ground plane and then have decoupling caps right underneath the ic on the opposite side of the board

If you are just doing two layers, then it is best to connect the EP, assuming there is only one ground (not separated analog and digital grounds), to the GND pins as well as place decoupling as close as possible to those pins.  I wouldn't recommend routing the GND out any other ways (out a corner of the package) just because this can get pretty hairy and is not the most ideal in regards to parasitics.  

If you do have separate grounds, make sure to connect yours to a signal ground, and NOT power ground.  Power ground is very very noisy.