This was prompted by this post here.
This image represents a 5x7 paper product. The gray area is the paper that would be trimmed away, leaving a 5x7 piece (outlined in red) that would ideally have perfectly symmetrical borders and perfectly centered design just as you intended / just as you see.
But there is no way to guarantee the piece will be cut precisely. If the paper is even just a little bit shifted in the trimming process, you'll still get a 5x7 piece but the design will be off.
If your outside border stops at the guidelines and the trimming is off, there's a chance the finished product will have the dreaded white (or whatever the base material color is) edge showing on one side(s) with the opposite side(s) being a thinner border because it's all relative... what's physically added to one side must come off the other to maintain the 5x7 cut size. [Left image].
If instead you try to play it safe by extending your border color as far as you can to the edge of the canvas and the trimming is off, you won't get the white edge, but the border will still still be off for the above reasons. [Right image]
Either way you go with an outer border, there's a risk that the trimming process will result in an asymmetrical outer border and white edges showing. And while this doesn't effect the actual size or width of any inner elements, it does visually shift them off-center as the whole design has been unintentionally off-set in the cutting stage.
I love the look of borders myself and even knowing the risk I designed business cards for myself with a left & right border like this:
I ordered them as both the regular card stock product and as the magnet version. The magnet version came out perfect as far as my eye could see; even with a magnifying glass the left & right sides had no perceptible difference. The regular card stock version however was inconsistent. There was a slight but still noticeable to the naked-eye difference in the size of the borders and within the 100 per order, some were more off than others.
Here's another product I designed for myself using an outer border:
I was really leery on this one because I thought that with the circle shape any accidental off-set in the trimming (and thus the symmetry of the border and centering of the content within) would be really noticeable but I guess I got lucky because all 12 or so I ordered looked perfect to me.
** I think it's worth noting that even without a border element there is still of course the potential for the trimming to be off. It's just that without a graphical border it's much less noticeable.
My point of this post was to try to illustrate what could happen with a border design. The trimming could be perfect, or it could be off, or it could be off but slight enough that anyone but you the designer won't notice. So while borders are always a gamble it doesn't mean you shouldn't ever use them. It just means you need to be aware of the potential result and decide for yourself if it's a risk you want to take depending on how much an off-set would visually effect your design.
