These size differences vary by very small amounts. For example, the difference between a 5.75 and 6 ring size is so so so so slight. Just eating too much salted popcorn can cause your fingers to swell more than a 1/4 size the next day.

Does going up half a ring size make a difference?

No matter what, always try to go a few sizes up so the ring can at least fit on your partner's finger for the proposal; it really kills the mood if it's too small to slip on. You can still surprise them, and simply bring it back to the jeweler later for resizing.

Is it better to size up or size down a ring?

Truthfully, if you're ever unsure of your ring size, it's always wiser to size up. It's much easier to resize a ring that's too big than it is to resize a ring that's too small, provided the ring can be resized at all.