I agree - don't listen to the quilt police. I did use diagonal seams on my 8" border and lo and behold ran out of fabric. So, I had to trim my border to about 4" since my remaining fabric was 4" wide. That way, at least I could finish the quilt. I was quite surprised how much fabric the diagonal join used! I'll use straight seams from now on.

I never do diagonal seams in the borders. #1 it wastes a lot of fabric and #2 it creates stretch where you least want it.

I have always made straight seams also. I am sure the other way would make it too eye catching. In fact I can never remember ever seen in done the other way.

I have done straight seams many times. I think it is up to the quilter

Go for the straight seam...you are the only one who will know.

put in cornerstones and if that isn't enough, add another block like the cornerstone in the middle of each border, or use three in a row of your blocks. There are a dozen ways to "stretch" your border fabric. Do what you need to do to make it work.

Straight border seams work for me. I only use diagonal seams for making the binding strip to save the "lumps" at the edges of the quilt I'm finishing. If you are afraid your eye will be drawn to a straight border seam, press it open so that it will pretty much disappear when it is quilted. I will admit that I do try not to place any border seams opposite one another. Equally placed border seams seem to draw attention to themselves. If they are staggered, they don't seem to so much. Make sense?

Heavens yes, a straight seam with that fabric is fine. But what I have done in certain circumstances if I've had to piece a border that I would rather not have to piece, is to insert a piece of some of the patterned fabric I've use in the quilt, usually only 1-2" wide, and make it look like a "design decision" rather than a place where I really didn't want to seam the border. Also I've pieced several of the fabrics from the inside of the quilt that are about 1/2" wide and pieced to make a section about 2" wide and inserted that. It ends up looking interesting, and makes the seamed area part of the design.

I cannot find a reason not to use straight seams. I use diagonal seams on my bindings to eliminate bulk, but that should not be an issue on a border. Sometimes I miter my borders to enhance a design, but even that is optional.

Another straight seamer here! The other thing I like to do to stretch the fabric when I don't have quite enough, as others have said, is to insert blocks into it. My favorite way is to put extras in the corners diagonally opposite from one another: top left and bottom right I might do cornerstones and then add 2 more of that same block to each side of it with a same sized section of border fabric between them. Or a row of 3 or 4 of the cornerstone blocks starting left top and going down, then right bottom going up.
I really like doing this with kaleidascopes when I can then use some of the smaller scraps in the cornerstones.