If you are really needing to create an AABB, you will have to generate it based on the largest X,Y,Z value of the post scaled, translated, and rotated model. Its more important to do it against the post rotated model as you need the X,Y,Z based on where it lies after rotation, the result could then be scaled and translated. This will still need to be done each animation step as this is going to change where the BB is going to lie based on the location of the verticies changing each step.
You probably shouldn't bother with the generation of the maxes prior to transforming the model if you're not concerned with the OBB, so once you've transformed it calculate based on the same process that you did when calculating the 'OBB' the first time.
If you calculate the model maxes prior to the translation/scale/rotate you will get the maxes based on the model only in model space, and applying the same transformation will result in a OBB in world space.
Just to consider, below is an example of where an AABB is going to give you something that doesnt look right for the volume taken up by an object.
The blue boxes are the same size, the AABB box has been cast in red on the box on the right.
The OBB for the blue box on the right will be the same space as the blue box.
If you go to investigate collision detection later down the track, AABB's are great for quick culling for proximity, OBB's create better accuracy for your final pass.