Does anyone have experience using USPCC die line templates? I have submitted art that extends right up to the dotted lines on their templates (i.e., past the safe zone), but that is within the (solid) die line. Do folks know how important it is to keep image within the safe zone? Seems like a very large safe zone to me.
The tighter you go in terms of a margin's width, the greater the chance that registration errors will ruin some portion of your print run, perhaps even a large portion.
As stated, USPC focuses on getting as good a registration as possible on the card backs. These days, the fronts are almost never completely centered, but that's less critical - an off-center back can make a two-way back design into a one-way back. As much as customers love getting really thin borders on their card backs, it's a dance when dealing with USPC, getting them to accept the art then having them tell you that they accept no responsibility for registration errors if your margins are that tight.
For the faces, stay within the border for everything but perhaps the indices, and even with those, don't go too close to the edge. Better to have a bit of wiggle room than an index that's cut in half, with the other half appearing on another card or falling off with the deck sheet's trimmings.
Consider using a different printer if you want better registration. Expert and Legends PCCs both accomplish some very good work in those aspects of a print job. Of course, there's still a limit, but they might be able to more accurately line up faces to backs, allowing you to go tight on faces as well. You'd have to talk to them to make certain, but it doesn't cost anything to ask questions.