This may be a really wacky question
 

I was taught that when sewing, if one of the fabrics is just slightly longer, put it on the bottom and the feed dogs will ease it in. I've seen this work, so OK. But...if this is true, how can we sew together two fabrics that are exactly the same size and the top one doesm'e end up longer at the end of the seam? Either the feed dogs pull in more fabric or they don't. Thanks for any input...this is really bothering me.

that is how borders end up ripply. The prepared border needs to fit perfectly and be fed together or there will be fabric left over at the other end

for seams, a yard-ish, I pin in the middle and be sure as I feed the 2 layers of fabric pinched gently between my fingers
Shorter than that I made sure they are even lengths going in and pinch gently as they go through, checking to make sure they are together at the rear end as I go.

even a 10" block seam can get off if not kept in check.

I was instructed to hold the fabric up about 6-8 inches to allow the feed dogs to ease the longer bottom fabric. If laying flat it does not ease it.

When I sew long seams like borders, I pin, glue, press then sew.
Works like a charm. I also starch all my fabric.

Thanks for asking. I have already learned from the responses.

I pin all borders in center at ends and half way between end and middle. I hold in between pins. Works perfect for me.

I secure each end of the seam and the middle for anything over 8 inches.

Some machines have even feed feet that work better than others. Both of my Brother's will pull a little bit extra on the bottom if I don't hold them together snugly. And by a little bit, on a 4 inch seam, I will lose a 16th of an inch. Not much, but to me it makes a difference when I am trying to keep everything square. I watched Jordan of Jordan's fabrics sew on a commercial Consew and her machine pulls fabric through beautifully. Some of the Pfaff's have even feed feet. I haven't used one so don't know if they are really better or not. Oddly, when paper piecing, the seams will stay even without me holding them snugly. I have to believe it is because the paper by itself will never give any, whereas fabric will.

For borders, I pin, pin, pin. I mark and pin the top and the bottom and then at the center point, at the 1/4 point on each side of the center and still hold my material snugly. I add pins if I have more than a foot between pins. I have had only 1 wiggly border when doing this and I believe it was because I cut my border on WOF which gives it more pull/stretchiness. I have learned to cut my border material first along the selvedge edge so that the borders don't have any stretchiness in them. If my blocks seem to be stretchy, I will staystitch the enter border edge before adding the final border.

This problem is created as the feed dog pull the bottom fabric, while the top fabric has the drag of the pressure feet.
The straight stitch Brother Nouvelle 1500S has made accomodation for the problem with the pen just behind the pressure foot, that coordinates with the feed dog. It will sew a long seam without the fabric shifting.

Perfectly reasonable question. If you pin, glue, baste 2 pieces together , they will (should) feed together exactly the same. But, if you have one that is slightly longer, the rule of putting it on the bottom (phrase 'bottoms are bigger' helps to remember) will allow the feed dogs to assist you in easing in the bit of difference.