No problem, you can also leave the idle mode by the timer and then decide (after debouncing) to go into idle again or not.
It cost no significant higher current consumption, but save additional hardware (CD4082).

If real current saving was an issue, then I suggest 8051 derivates with wake up on keyboard interrupt and go into power down mode, if nothing to do.
After wake up use the timer interrupt to read and debounce the keypad.
Then enter power down mode again, if the key action was done or if only bouncing (noise, electrostatic discharge) was seen.




Then he was the first guy, which was able to write a working application without debouncing mechanical keys.


Peter