Printers may sometimes need to preserve the deckles on handmade or mouldmade paper but this can be problematic in registration. A not uncommon practice is to file away the deckle where the paper fits into the guides. This leaves a barely discernible registration edge that is even less so when the paper is dampened. This can be accomplished in a number of ways but to find the edge where the leakage (the deckle) begins and where the true edge of the paper thickness ends is quite difficult.
I was watching a student working on some deckle edged paper the other day and noticed he was using a small light and marking the paper with a pencil before making his cuts. When I inquired he showed me his device, a runner's mini LED (straight line) hat light clip on. He placed the paper with the deckle just beyond the edge of the table and ran the light along beneath the paper. The edging was perfectly revealed!