I might just have been lucky, but in the last two years I'd say, I've been to La Ronde several times where the lines were fairly manageable even on days where I expected crowds. However it is probable that there will be more people on a Wednesday, especially in the evening, as it's a fireworks day.
It is also true that if you go right at the opening you will have to wait in line for a while to get in the park, so unless you're pressed by time or really want to spend a lot of time at the park I would suggest you to wait one or two hours after opening to go.
If you have at least a Platinum membership you don't have to wait in that line as you can go through the membership entrance. I learned this when I went in May, and it REALLY helps as you get a good couple hours of no lines before all the regular pass holders and ticket people come in.
I visited earlier this season and queued up in the absolutely massive regular line, until I noticed there was a membership entrance that didn't have a single soul in it. So that definitely saved a lot of time. I just have the regular gold+ membership, and they let me through.