Transformer secondary switching and EMF

[e100]

When using a typical mains 240 to 12v transformer to power a resistive load, does it generate a voltage spike when you interrupt the secondary, or does the stored magnetic energy go back to the primary side?
On the web there's lots of info about inductive loads, but not much about inductive sources.
The application I'm thinking about is a low voltage trailing edge dimmer circuit for rectified 12v. Effectively PWM synchronised to the mains frequency. Do I need to plan for higher than expected voltages other than the 1.4 x RMS?

Mike