I would be inclined to put up the cage first. Then you can see exactly where to plant your fruit. Also, if you trodden on the soil inside the cage, you can loosen it again before planting.

Agree with Andrew erect the cage and then plant the fruit and if you are growing raspberries remember to leave a metre of space, or more, between the rows.

Thanks for that, it's what I had thought, too.
Now, my next question, please!
I would like to plant raspberry canes and gooseberry, black currant and red currant bushes in the cage. If I were to plant two parallel rows of raspberry canes running the length of the cage, then put in a parallel path of say 2', (is 2' wide enough?) and then plant the bushes on the other side of the path, parallel to the raspberry canes, how wide do you think the cage would need to be?

As I already stated you need at least a metre between your rows of raspberries. Beyond that it depends how many canes and bushes you plan on planting - you need about half a metre between canes.

The paths in my fruit cage are 18 inches wide. As long as you leave enough room so the fruit bushes don't grow over it, this is plenty wide enough

However be prepared for the raspberries to send up runners into the other bed - mine certainly sometimes pop up at least two feet away from the parent.

Yup that is our experience too.