I am using ble_app_beacon example and I want to know if DC/DC is enabled by default in the app or should it be enabled like in nrf51 series?
The DC/DC is not enabled in the SDK examples. You can enable it using sd_power_dcdc_mode_set().
But where do we have to put it before advertising or after that?
You can enable the DC/DC at any time after you have enabled the SoftDevice. For example, referring to the SDK 15 BLE examples I would say that it make sense to call sd_power_dcdc_mode_set() immediately after the call to ble_stack_init().
I tried to call sd_power_dcdc_mode_set(NRF_POWER_DCDC_ENABLE); right after the ble_stack_init() but sometimes it makes my application crash. Is it because it's called too soon after the initialization call? When I have the debugger connected the application doesn't crash but without it, it does. It suggests a timing issue.
I had a case where I unknowingly enabled DC/DC, and had the same issue where the program would randomly crash without debugger, but would work fine with debugger attached. In my case it was because I didn't have the necessary external LC filter circuit.