I have parkerized several bolts without any additional heat treatment, and I have not had any problems in thousands of rounds. I bead blast them, park them, oil them, and shoot them, in that order. Over 2K rounds on one that I am aware of and no problems. I think you are worrying about a non-existant problem. If you feel better, bake them at 200 to 215 degrees. As an old tool & die maker that has heat treated a lot of steels I can't imagine 215 degrees having any effect what so ever. Now 375 degrees is getting up there where you can start changing the metalurgy and could possibly start warping things. I would be careful about that.

You can do more for safety by properly lapping the bolt in to the lugs in the receiver so that both bolt lugs share the load equally. A lot of guys swap a bolt, check the head space and figure they are good to go. The headspace may be check good, but that headspace may be based on one lug resting on a high spot.