It doesn't matter to me whether they are rounded up or down, when I play a 4.5 that's a 5 on the card and I feel like it should be a 4 for me, I think of it that way.
There's a little 9 holer I played 4 or 5 times this winter when it'd get to the mid 50s, and there's a 470 yard par 5 there where I hit a sort of half-assed drive but followed it with beautiful 2 iron about 18" from the hole. The guy I was playing with (just someone I joined up with) was like "wow, an eagle" after I tapped it in and I told him I thought it was really a birdie because that hole wasn't a legitimate par 5 for me. Which I suppose would be true in the summer, but a 470 yard hole that's uphill the last 250 yards, on soggy ground, into a bit of a breeze on a 55F day probably was a par 5 for me that day, but I said what I said without really thinking about all that.
If I've got a short 5 I can see myself falling into the trap of thinking of it as an eagle opportunity -- thinking of it as a birdie opportunity would be fine, but when you think eagle on the tee you often find yourself needing to make a good putt to save par, or worse! But when I've got a tough 4 I'm not thinking birdie on the tee, I'm thinking par, I wouldn't think birdie until I've got it on the green with a makeable putt. So maybe mentally altering the par of a 4.5 listed as a 5 on the card is my way of sort of protecting myself against trying to push too much.
As far as Dan Hermann's comment, I like that way of thinking. At Saddleback Ridge, one of my local favorites, the 15th is a 549 yard par 5, but you tee off from way up high on the ridge and it is with the prevailing wind, so its reachable for me everytime with as little as a PW once. The hole that follows it is almost as long, but its into the wind and back up that hill, so its definitely a real three shotter most of the time. They'll often move the tees up about 30 yards so it plays more like 515 on really windy days, but I wonder what it would be like if it were more like 445 and listed as a 4 on the card. When the wind was really strong it would really play like a 5 but you get the previous 5 playing like a 4 to make up for it. It would be an interesting test of a golfer's mind. You see that sort of thing in Scotland and Ireland a lot.