I don't know of any reason you *have* to leave a border other than what you said, to avoid having to cut into the design, but I've also seen pics where folks have cut right *to* the design to leave no border.
I have done both. I can say, when I do not leave a border, it does look like the color of the ink comes out slightly from behind the paper when the polymer melts. It never affect the image ever. But I imagine the polymer melts slightly past the paper edge when there is no ink to highlight that it is happening, so I always use a piece of teflon.
I remember the first time I pressed a navy blue bordered imaged and I had trimmed to the design, I could see the slightest line of navy around the paper before I peeled. I wondered, but was glad to see it affected nothing when I peeled.
That is all I have ever seen that I can think of to say about it. Maybe someone else can comment more on it. I wouldn't hesitate to leave a border, nor would I hesitate to not leave a border.
In gemini's example of JPSS on ash grey, I thought in a case like that I would cut all the way *to* the design (I love that tshirt, gemini... I think it's really enhanced being on a ringer tee. Love it!)
You could always try both methods, Spank, and see what you like or dislike, if anything. I found that probably it would more depend on the job than my preferences bc I found no real issues for or against.
