I came across this great video of classical guitarist Scott Tennant talking about nail shape. I think it applies equally as well to steel string fingerpickers.
There are alot of picking hand approaches to fingerstyle guitar. Folks pick with no nail, natural nails, artificial nails, or fingerpicks and all sound great. I think it comes down to personal preference and what sort of tone you are looking for. Having started off with classical guitar my preference is for natural nails, and thankfully mine are strong enough to hold up on steel strings. I have generally gone with Shape #1 in the vdeo, where the nail roughly mimics the contour of my fingertip. It has worked well enough for me over the years, and I think my tone is decent. However, lately I have been wanting to experiment with ramp shape #3 in the video. My friend classical guitarist William Bajzek gets a killer tone with this nail shape, both on classical and steel string. Really warm, thick, and rich. I think I may let my nails grow out a bit and give this shape another go.
Its always funny when you get a bunch of grown men together who play fingerstyle guitar and they invariably talk about the shape and maintenance of their fingernails.