If you have a good hardened tip blade it will saw a few nails before it
is trashed. I won't like it, but it will do it. I can't see any
permanent damage to the saw, just noise, maybe smoke from the blade,
but usually it is over before you realize it. I would definitely use a
guide, even just a 2 x 4 trim screwed down or someone standing on it.
I'm not steady enough to make a long straight cut freehand.