Quick nail anatomy lesson!
The area of living skin around the top of the nail often referred to (improperly) as the cuticle area is actually called the Eponychium - also referred to as the "nail fold". The cuticle is actually a very thin layer of dead skin on top of the nail that is often scraped and removed during a manicure (it should only be partially removed, not completely removed). The eponychium along with the cuticle help to seal the nail from bacteria. The area under this and below the nail (where you can see a white crescent) is where the nail is actually formed and is called the matrix. Should the matrix become too damaged, the nail will no longer form.
It's not good to damage the eponychium. It's an area highly prone to swelling up like a balloon - rushing white cells to the area - to help prevent bacteria from entering the nail bed and permanently damaging the matrix. You know, like... you get a scratch anywhere and - no biggie - but you get a scratch right along your eyelid and all heck breaks loose! Natural defenses and all. Be careful there.
I don't know if you'd build up callouses there or not, I kind of think not though. Possible infection in that area would cause a lot more pain and possible trouble down the road. If you're playing that rough, you're scraping off small layers from the top of the nail too. Eventually this could cause brittle nails, splitting, flaking, etc - making it difficult to grow the nails long enough and keep them there for finger picking. I think - if you're doing a lot of strumming and hitting the strings with the tops of the nails - it might be a good idea to plop on a protective layer (Look for a nail ridge filler - it's generally fairly clear and matte).
If you do opt for a Fire Engine Red topcoat lacquer, you'll be able to see very quickly how much damage is being done to the top of the nail as the polish gets scraped and scoffed off. I try to limit myself to just a few songs that I strum without a pick, for the health of my nails (and rarely wear polish because it doesn't look good at all after an evening of guitar playing).
Interesting fact: It takes about 6 months for the body to repair the typical damage done by one poor nail service (ie, acrylic nail surface preparation generally includes filing the nail surface and the "cheapo" places tend to over do it. Then women will also often peel the acrylic off once it starts to lift - taking additional nail surface layers off in the process). These are your claws. Protect your claws.
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