Sorry folks, I was led astray by the other reprobates!
Going back to deck fastenings I think we've all learned more than we ever dreamed possible about these fixings.
To summarise, it seems to me that:
1. On a well maintained vessel they would be invisible under the plugs
2. On a badly maintained vessel with the plugs missing they would be virtually invisible at most scales, and vey small at the larger ones. They would be smaller than rivet heads for example.
3. If you are depicting the vessel as in need of maintenance then the rest of it should be in keeping i.e. dents in the hull, bent fittings, rusty patches everywhere etc. There is a good example of this ashore at the place I keep my boat, a small and very battered steel fishing vessel - I'll get a photo next time I see it.
4. As Bryan says, it's probably more important to get the planking butt lines in correct sequence to aid authenticity.
Colin