Yes, it is true. You shoul avoid to put the watchdog timer reset code in the timer interrupt function, otherwise the watchdog is like an otiose object, you reset it again and again just for feeding it.
If you really want to use it, pls put the reset code in your main loop or a loop block waiting for events. At beginning you can put the reset code as many as the mcu can run. Then try to remove them one by one until the mcu can not work. Then back one or two step you wil get the best point.