Unless you put something in the barrel to raise the pressure in the gun, the flame from a percussion cap will NOT be hot enough to burn out most oils. All it does is foul the flashchannel( the small hole in the bolster between the base of the nipple, and the powder chamber.
The components of MODERN percussion caps are not Corrosive. However, they attract moisture, which will rust the flash channel, and bore. Clean the flash channel and bore any time you fire a cap, powder charge or no, and will be letting the gun SET for any length of time.
I recommend flushing the barrel and nipple out with alcohol before leaving home. Then, put a very thin coat of vegetable oil on a cleaning patch, and Lightly lube the bore for transportation. Leave the flash channel, and nipple dry( save for a drop of oil on the threads of the nipple before screwing it into the bolster or drum).
If you will be driving DAYS to get to a hunting place, take the alcohol with you( You should have it on hand for any number of uses, anyway), and do the flushing the nite before you take to the field.
You can use alcohol in a cleaning patch to remove that thin layer of oil from the barrel before loading your powder charge. Give the alcohol time to evaporate, or run a dry patch down the barrel to remove any excess alcohol you may inadvertently put down the barrel. If you use a vegetable oil/wax lube mix, you can leave the thin oil/wax mix in the barrel and just load your powder and PRB. The oil/wax mix lube will keep fouling soft after your shot, to make it easier to wipe out with a spit-dampened cleaning patch. If you are hunting in warm weather, follow any dampened patch with a dry one, or two. Read your cleaning patches. They will tell you how wet, or dry the bore is, as wet residue comes out appearing shiney, or "slick". If the barrel is dry, the residue will appear gray on the cloth patch.
Load your gun in good light, rather than fumble around doing so in the dark. I have hunted with my Mler in temperatures as low as 10 degrees F, and as high as 85 Degrees F. The relative humidity and precipitation makes a huge difference in how you keep the gun dry.
Firing a cap off ONLY allows you to SEE ( by watching a leaf or blade of grass move) that the channel from the nipple to the barrel is unobstructed, and the bore has nothing in it to prevent a flame from traveling its full length, and out the muzzle. A bare ramrod down the barrel answers the second half of that question when it clicks when the metal jags hits the breech. Alcohol pouring out the nipple answers the first half of the question, much better, and without dirtying up the flash channel. :hmm: