I just do it on the outer edge, especially if there are a lot of narrow seams. If I have to piece a long border, I'll typically reinforce just an inch or so either side of the seam, but I really prefer to use full length cuts when I can.
When I'm piecing, I'm putting the blocks together fast enough that the seams are interlocking and reinforcing each other, plus the size of the pieces are small so there isn't a lot of drag on them.
My concern is in folding/unfolding and spreading out the top for viewing or measuring or whatever until the top is quilted down, which in my case can be years before it happens. If I'm on a deadline and it's going directly from the machine to the long arm, I may or may not reinforce the edge. I still will if there are seams every 2" or less. I might with 6" seams, but I don't typically bother to do straight grain fabric.