Besides the fact that using the MODELVIEW matrix is wrong if you want to go to world space (see my post above) ... it depends on which kind of bbox you want. A so-called axis aligned bounding box (AABB) is, as the name says, axis aligned, usually with the world axes. This kind of bbox undergoes no rotation. Instead it has to be recomputed by finding the maxima and minima after scaling and rotation has been done to the model. On the other hand, the object bounding box (OBB) will be scaled and rotated like the model. Both are ways are valid.
The way I generally approach it is to give the box its own coordinates, and apply transformations to those manually using vectors, Matrices and quaternions.
Right after setting up your transforms, you can call glGetDoublev(GL_MODELVIEW_MATRIX, double array). With that matrix you will have to manually loop through your Box vertices and do matrix multiplication.
I had considered this, but if I were to generate the bbox, and then apply the transformation/rotation/scale to it manually, a rotated model, would result in a rotated bbox. I was under the impression that this was bad practice. Unless I am mistaken.
Alternatively, I could use the apply the matrix to the points in the model, and then calculate a bounding box, but since applying rotation requires trig, and it would be run on every single vertex on every model, each frame, I imagined it would absorb ridiculous resources. Again though, I could be mistaken here as well.
Trigonometry is required just when computing the transformation matrix in the case that the orientation is given in angles. After that, rotation is part of a matrix / vector product which involves no trigonometry.
There are other kinds of bounding volumes that may serve your needs as well. E.g. a cylindrical bounding volume may be used if the model will mainly be rotated around one axis only (e.g. character avatars). Or you may want to approximate an AABB by building an axis-aligned box from an OBB.