I would recommend PAT testing much more frequently than yearly.
The big danger is damage to leads from twisting, pulling, chafing etc which could lead to (no pun intended) an electric shock risk from damaged cable, or from a broken earth conductor.
The other risk is cables being pulled from their grips at either the plug or the machine.
At a very minimum, ideally every time you use the machine, or at least once a week, you should:
1) check that the lead is secure at the plug and machine
2) run it through your fingers BEFORE PLUGGING IN!!!!! to feel for any kinks or cuts
3) check that the machine, and plug, is undamaged.
AND ALWAYS, ALWAYS, USE A RCD ADAPTOR IN THE WALL SOCKET!
That £10 adaptor from B&Q could save a life.