Must I continue to train troops if my APC are full, is there any benefit training more, if I’m attacked..

This depends on how you play. Having spare troops to replace ones that get injured can be useful.

If you are a farmer for example and at lower levels then you may not be doing too much attacking. And low level troops are less effective.

A raider will need plenty of troops to fight with.

More troops also means more food/fuel consumption so you need to bear that in mind with your available production levels.

Any spare troops you have will also fight to defend your city if you are attacked. They fight after your main APCs have been used.