Sue, in general CRA's rules for keeping records are the same for electronic ones as for the old paper-based ones. They include:
-- Information must be complete and accurate,
-- Information must be able to be located if needed,
-- Records must be kept for the statuatory length of time (usually 7 years)
-- Records must be easily readable.
In addition:
-- Records must be stored safely and securely.

Of course these requirements are easier when you burn each year to a CD or DVD and make a cou0le of copies (Try that with your paper files )