Yes, they get rounded and have (on both sides) the +–1. (I think, would conclude)
You can notice that well in serbia, wether you attach the corps to the hq or not, same as west front. You have a (mediocre) lvl 5 gen there, you unit may forecast 3:2
Switch to the lvl 6 or 7 general it doesnt change. Use the lvl 8 general and it turns into 2:3
In my gut feeling in the example, the lvl 5,6,7 generals achieve the same results, only (in that example) lvl 8 has an effect. But it could of course be that the lvl 7 generals tips it slightly more, that he more often adds enough for a +1 damage.
But i would like some clarification aswell. Of course the combat values have long decimals with all the -30% from trenches, morale and readiness, generals.
Question 1: do these values get rounded down and up? Meaning wether you attack entrenched verdun with a strength 1 garrison or a strength 10 corps, both show that you will deal 0 damage. Is that 0 the same value, meaning the almost dead garrison and the full corps have the same chance to deal the extra +1 damage?
Question 2A: the +-1. Is it just "-1/0/+1" or all decimals in between eg +0.5/-0.777
Question 2B: If the bonus aut malus is a decimal and if there is rounding. Does the rounding happen before or after adding the bam?
1) It is just rounded, up or down depending on whether or not it is >= .5
2) Just "-1/0/+1"
3) The "-1/0/+1" is applied after the combat results are determined
Hope this helps,