The scant can make a big difference in HSTs (half-square triangles) and using a full 1/4" can lead to lost points.
I was in a swap exchange where the blocks had to be 12.5" and when I used my regular seam allowance, some blocks came in a bit smaller, so I had to resew them. Not fun! Now I spend more time measuring as I go so I don't have any do-agains.