Checkering is done after the stock is finished and checkering oil is applied, checkering oil is a thin mixture. If you get tru oil in the checkering it will fill up and look like crap.
I have used tape to protect the checkering from filling with finish and even used brownells release agent on fine checkering. But the best was silly putty, It worked the best because it can be pulled out of the checkering as one piece leaving nothing behind. but if you are carfule and clean out the checkering with a soft tooth brush between coats you should be ok.